Andy Booth had given the Terriers a dream start after he capitalised on a poor back pass across the Brentford area after Thorrington and Facey had combined to attack the Bees.
Booth turned and shot on a tight angle to beat Paul Smith at an awkward height - he fumbled the ball and allowed it to trickle over his line.
First blood had been drawn and the opening stages of this game were electrifying as both sides attacked freely.
Brentford were not going to give up so easily, though, and they soon got back in their stride with an equaliser from Lloyd Owusu after 14 minutes.
Paul Evans, the Bees captain, had already had a range finding free kick whip past the wrong side of the woodwork but he made no mistake after Jerel Ifil was adjudged to have blocked Owusu in a dangerous position.
Evans stepped up and bent a great free kick into the area for Owusu to glance home unmarked beyond Margetson.
The home support got behind their team and the Terriers began to struggle under pressure from Brentford who had again double marked Thorrington and were keen to use their physical presence to bully Town off the ball.
The Bees had a further four free headers in the first half, Ivar Ingimarsson was very effective and had one effort cleared off the line by Chris Holland, as Town lost their way with Thorrington (closely marked by Ijah Anderson) and Facey (who had one deflected effort cleared off the line by Ingimarsson) in particular struggling to escape their markers.
Not even the loss of Martin Rowlands seemed to faze the Bees, whose replacement, Kevin O'Connor was set to break Town's hearts after the restart.
With the scores level at half time, the Terriers could start again after a torrid first half.
Less than a minute after the restart, however, Town's second half nightmare began.
O'Connor had taken the ball deep into the Town defence and was seemingly fouled.
Town's defence stopped momentarily, expecting the referee to blow his whistle, but he didn't, and O'Connor slipped the ball to Lloyd Owusu who revelled in the space and time he had with which to curl the ball, unimpeded, past Margetson for 2-1.
The stuffing seemed to have been knocked out of the Terriers as they had to try, ultimately in vain, to equalise.
Owusu was causing all sorts of problems, though.
He might have finished the game twice as he spurned chances for his hat-trick by hitting the side netting on the hour and then misdirecting his header with just Margetson to beat 5 minutes later.
The Terriers didn't have the same penetration.
Without the suspended Leon Knight and injured Danny Schofield they were a one dimension side in the second half and luck seemed to be against them too as a hard won and clear penalty claim with 5 minutes to go was ignored by both the referee and linesman.
Kenny Irons' corner had been flicked on by Delroy Facey but Ijah Anderson clearly handled the ball in the melee but nothing was given and Town's Millennium Stadium vision drained away.
Read Mel Booth's match report on Thursday. Have your game on the game and Town's season as a whole in the messageboard below. Please leave your full name and location if you wish your comments to be considered for publication in the Examiner.
Brentford (4-4-2): Smith, Anderson, Ingimarsson, Powell, P. Evans, Owusu, Rowlands (O'Connor 35), Dobson, Hunt, Burgess (Boxall 69), Sidwell
Subs not used: Gottskalksson, Williams, Theobald
Booked: P. Evans
Scorer: Owusu 14, 46
Town (4-4-2): Margetson, Jenkins, Gray, Ifil, G. Evans, Thorrington, Irons, Heary (Hay 73), Holland, Booth, Facey
Subs not used: P. Evans, Moses, Mattis, Delaney
Booked: Thorrington, Ifil, Hay
Scorer: Booth 3
Referee: Graham Barber (Tring)
Match points follow on subsequent pages.
93 mins: A handball on the edge of the Brentford area was not seen by the referee.
92 mins: Everyone was back for the Bees as Town pushed one more time.
90 mins: Seconds left but Brentford still had the ball. Thorrington couldn't get past. Chris Hay and Paul Evans squared up after Evans appeared to head butt Chris Hay - no red card was forthcoming for the infringement - 3 minutes of added time were left for Town to score.
89 mins: Town inched upfield but had to battle every step of the way. Facey's unguarded push gifted the possession back to Brentford.
87 mins: Town were finding it hard to break down the Bees as the seconds ticked away.
85 mins: Irons' delivery for the corner appeared to be handled in the Brentford area by Anderson but the referee was unsighted and the lineman didn't move!
84 mins: Chris Hay's shot was deflected just wide and perked up the travelling crowd.
82 mins: Brentford looked comfortable as Town struggled to find their rythym with time running out.
80 mins: Thorrington and Jenkins experienced frustration as they couldn't find a way past Anderson.
79 mins: Facey was thwarted by Powell on a run to goal.
77 mins: Booth won a free kick after Ingimarsson climbed all over him.
76 mins: Owusu missed his near post kick from a Bees set piece.
75 mins: Thomas Heary went off as Chris Hay came on - Town HAD to go for goals but they retained a 4-4-2 formation with Chris Holland coming inside and Chris Hay taking up the left flank.
74 mins: Jenkins and Thorrington were making inroads down the right but Paul Evans hooked the ball away. Chris Hay was ready to come on.
73 mins: Chris Holland's shot was saved after Andy Booth laid the ball on.
72 mins: Evans gave the ball away in the Brentford half after a promising run. O'Connor hit along ball for Owusu but Ifil put his foot in for a great tackle.
71 mins: A great tackle from Ifil was strangely penalised - Owusu raced up to Ifil and reacted badly as Jerel Ifil was booked. Kenny Irons argued the young defender's case to no avail.
70 mins: Ben Burgess went off and was replaced by Danny Boxall as the Bees decided to protect their lead.
69 mins: Town were struggling to get service to Andy Booth as Brentford pressurised them.
68 mins: Thorrington was being double-marked.
67 mins: Thorrington fell over just outside the area under pressure from Anderson but the free kick was not given despite calls for a penalty from watching Town fans.
65 mins: Jenkins challenged Burgess for the ball and conceded a throw-in.
64 mins: Owusu got goalside of Ifil and met the diagonal ball but couldn't direct his header with only Margetson to beat.
63 mins: Gray cleared under pressure from Anderson. Kenny Irons was in danger of being booked after continually talking back to the referee.
62 mins: Heary's superb tackle allowed Irons to play the ball upfield but Town were slowly running out of time.
59 mins: Owusu rounded Margetson but could only find the side netting after a horrendous back pass by Jenkins!
57 mins: Facey won a header as Town laid siege to the Bees area.
56 mins: Facey chased a lost cause and won a corner.
55 mins: Lou Macari was furious that a foul on Ifil had been overlooked.
54 mins: Irons' tricky free kick was gathered by Smith with Booth waiting.
53 mins: Anderson's cross came off Gray and was eventually cleared. Irons went upfield and found Facey but his final cross was terrible with Booth and Thorrington waiting in the area.
52 mins: Burgess nutmegged Gray but Jenkins made the clearance as Brentford began to show off.
51 mins: Ifil and Gareth Evans cleaned up after O'Connor ran upfield.
49 mins: Brentford were lifted and the Terries were also stung into action as the game picked up.
48 mins: Facey's cross was met by Thorrington but Smith was ready for it.
46 mins: The Town players stopped as they thought a free kick had been awarded for a trip on O'Conner who had passed to Owusu. Lloyd Owusu carried on and planted a curling ball into the back of Margetson's net!
Town grabbed an early goal in an electrified early period as Andy Booth capitalised on a defensive howler to score in the 3rd minute.
Brentford wouldn't lie down, however, and Lloyd Owusu was unmarked as he headed home the equaliser on 14 minutes. Brentford had 3 further free headers and were very dangerous on set pieces as Town struggled to retain possession of the ball against the Bees.
46 mins: Margetson came then stopped, hesitating, as a teasing ball came in - the resulting header was wayward. 2 minutes of added time were indicated.
45 mins: Anderson's long throw was repelled by Jenkins.
44 mins: Ingimarsson tried to trick his way past three Town defenders out overdid things and lost the ball.
42 mins: Burgess' half chance was well covered by Margetson as it sailed over Margetson's bar.
42 mins: Holland turned but overran the ball.
41 mins: Burgess pushed Heary after the Irish midfielder had nicked the ball.
40 mins: Thorrington went upfield with support from Jenkins but the American ran into a wall of defenders long after he should have crossed.
39 mins: Hunt's cross nearly fell for O'Connor but Margetson was brave and cleared.
38 mins: Powell kicked the ball away long after a free kick had been given - and he earned a booking for his troubles. Dissent meant that the ball was moved forward 10 yards.
37 mins: Rowlands was replaced by O'Connor for Brentford.
36 mins: Thorrington tugged the shirt of a Brenrford player was looked set for a booking.
35 mins: Town were being frustrated as Facey gave the ball away in forward positions.
34 mins: A period of pressure play saw both sides tussling for possession.
31 mins: Facey raced Ingimarsson towards the Bees goal but the Icelander won out.
30 mins: Ingimarsson skied his effort from Brentford's long diagonal free kick! It was a fourth free header.
29 mins: Facey's shot from Irons' corner was deflected but blocked on the line by Ingimarsson!
28 mins: Booth's flick-on fell for Chris Holland who squared the ball across the area and Facey was robbed by Anderson who punted the ball over his own bar.
27 mins: Irons' free kick from 35 yards came off the ball. Jenkins put the loose ball in but Dobson cleared.
26 mins: Hunt delivered a high floater which was no threat to Margetson's goal.
24 mins: Ingimarsson's free header was Brentford's third - thankfully it didn't go in but Town weren't learning their lessons.
22 mins: Jenkins was down after a tackle and Brentford sportingly kicked the ball out of play.
21 mins: Booth's lay-off fell for Sidwell who cleared.
20 mins: Owusu's nudge took Hunt's effort wide.
19 mins: Facey was in space but couldn't get a shot in as Brentford converged on any Town threat. Ifil wasted his chance.
18 mins: Thorrington's cross was headed away by Rowlands but Thorrington retreived and crossed for Facey whose flick on didn't reach Booth.
18 mins: Ingimarsson's header was cleared off the line by Chris Holland!! Hunt's subsequent corner was less dangerous.
17 mins: Brentford won another corner - Paul Evans stepped up...
16 mins: Thorrington nipped past Anderson and raced to retrieve the ball before it went out for a goal kick but was inches too short.
14 mins: Lloyd Owusu scored the equaliser at Griffin Park after the set piece went brilliantly for the big man. His free header was glanced into the back of the net beyond Margetson - the terraces erupted into song.
14 mins: Ifil was penalised for blocking Owusu and conceded a dangerous free kick. Paul Evans stepped up....
13 mins: Thorrington tried to win a corner after a crossfield move by Town but didn't get the deflection off Anderson.
12 mins: Paul Evans lined up a free kick for Brentford in front of a 5-man wall and whizzed his shot just wide of the top corner and over the bar.
11 mins: Ifil put the ball out of play as the Bees attacked.
9 mins: Booth was elbowed in the face and won a free kick.
7 mins: Rowlands was blocked by Jenkins after he had foiled Heary in midfield. Town were floundering under intense Brentford pressure and were having trouble keeping possession.
6 mins: Town were giving the ball away as Brentford pushed on. Ijah Anderson's buzzing shot was well saved by Margetson.
5 mins: Poor control let Hunt down as a shell-shocked Bees side tried to get back into the game. Paul Evans tried to stroke the ball toweards goal but was found wanting.
3 mins: Andy Booth turned and scored a wicked opener from a wide angle with a rising shot. Paul Smith spilled the ball at head height and watched in horror as it tricked over the line!
2 mins: Paul Evans' corner was poor and Gareth Evans tackled and won the ball. Thorrington took the ball upfield and tried to find Facey but the ball was intercepted and the Bees defenders attempted a backpass across his own area which backfired horrifically.
2 mins: Hunt beat Jenkins and won a corner off Heary after a poor free kick by Evans was given away for a Bees counter-attack.
1 min: Brentford launched the ball upfield as they continued where they left off from Sunday.
With seconds to go, Griffin Park was alive with noise as Town prepared to kick off.
The Town team made their way on to the pitch to a rousing reception from the travelling support - they sprinted over to salute the fans who had made the trip south.
With less than 15 minutes to kick-off, Griffin Park was filling up quickly.
Steve Jenkins returned to captain the Terriers at the expense of Adrian Moses, who in turn displaced Chris Beech on the bench. Nathan Clarke missed out with a knee injury.
Lou Macari's men were facing an unchanged Brentford side at Griffin Park as they sought to book passage to the Millennium Stadium.