Sherlock star Martin Freeman's new look in BBC2 comedy Brian Pern: A Life In Rock

In the first episode of the show, which follows an earlier BBC4 series based on the character, Freeman plays a younger version of Pern in Stowe Boys – a West End rock musical about the band with a director played by Kathy Burke.

Pern (played by Fast Show actor Simon Day) is, of course, the former front man of the fictional progressive rock group Thotch – reuniting with his band mates for the musical before he's unexpectedly arrested by Operation Bad Apples Officers for a crime he doesn’t know he has committed.

Joining Freeman on stage in Stowe Boys is Jack Whitehall (also playing himself) who's been cast as a younger version of Thotch keyboardist Nigel Havers’ Tony Pebble (pronounced “Pebblay”).

While filming the episode, Freeman was sporting a beard he grew for his stage portrayal of Richard III which was running at Trafalgar Studios at the time.

However, the comedy chooses to explain away his facial hair by insisting the actor was recording a fictitious biopic of the singers Chas & Dave. (Before you Freeman fans get excited, we must stress there is no truth to this whatsoever).

Rhys Thomas, who is esteemed by comedy fans for his star turn as spoof radio presenter Gary Bellamy in Radio 4's Down the Line, has known Freeman for a number of years, but he was still surprised that the Sherlock and Fargo actor was able to find time to film the comedy.

“I didn’t think he was going to say yes because he’s really busy,” Thomas told RadioTimes.com.

“He was doing Richard III at the same time and I think he wanted a bit of a laugh. He loves music as well. He just did it for fun.

“Martin that day had a really bad back. He had hurt his back doing Richard III and he really put all his effort into it. He is one of the busiest people in the world and I thought the fact that he could come and give an afternoon to this was really nice of him and he had a really good time.”

Brian Pern: A Life In Rock begins on BBC2 on Tuesday 9th December