KNOCK knock, Who’s there...? Remember that old joke when a certain character, an ever-changing Time Lord, would arrive on telly. A real UK cult hero.

Now the Doc is back again tonight with yet another new female assistant, Catherine Tate, by his side.

Several generations have now grown up with Doctor Who. And, of course, hidden behind the sofa whenever these sinister creatures appeared.

So who is Who?

He is a time-traveller known simply as “the Doctor”. And he is listed in the Guinness Book of World Records as the longest-running science fiction television series in the world.

Doctor Who first appeared in his famous blue Tardis, a 1950s blue police phone box, on BBC television at 5.15pm on November 23, 1963.

The series’ title theme was composed by Ron Grainer and it was intended to appeal to both children and adults. When Christopher Eccleston as the Doctor and Billie Piper as his assistant Rose, the format changed for the 2005 revival, with each series consisting of 13 45-minute, self-contained episodes - twice as long as the original.

Its revival, due in no small part to David Tennant stepping into the Time Lord’s shoes, has made the series the centrepiece of BBC1’s Saturday prime-time schedules.

Doctor Who has spawned spin-offs in multiple media, including the television series Torchwood and The Sarah Jane Adventures.

The relaunch of the programme has also seen Christmas Day special episodes broadcast between series, the most recent starring Kylie Minogue.

Each incarnation brings fresh ideas to the character.

Strangely, all that was known about the character in the programme’s early days was that he was an eccentric alien traveller of great intelligence, who battled injustice while exploring time and space in his unreliable old time machine but with a companion in tow.

Like all time lords, the Doctor has the ability to “regenerate” his body when near death, allowing for the convenient recasting of the lead actor, although apparently this can happen only 12 times.

When Sydney Newman commissioned the series, he specifically did not want to perpetuate the cliche of the “bug-eyed monster” of science fiction.

However, monsters were a staple of Doctor Who almost from the beginning and were popular with audiences.

The Daleks - created by writer Terry Nation as an allegory of the Nazis - are perhaps his best-known adversaries, but he’s come up against a multitude of baddies including the Cybermen, the Autons and Slitheen.

Now Doctor Who is a sexy, clever, fun and must-see series and every Doctor has played his part in making this a pure TV classic.