1. Keira Knightley.
When did she first appear? Season 11, Episode 39: "Swan Song", in 1995.
Who did she play? Sheena Rose, a 10-year-old thief who is found to have a valuable collection of old coins in her backpack.
Where do you know her from? Pirates of the Caribbean, Pride and Prejudice, Atonement.
3. Denise Van Outen.
When did she first appear? Season 14, Episode 117: "The Personal Touch", in 1998.
Who did she play? Melanie Lehmann, who calls the police after being robbed by a gang but fails to mention that she's dating one of its members.
Where do you know her from? The Big Breakfast.
4. Craig Charles.
When did he first appear? Season 11, Episode 124: "Honeypot", in 1995.
Who did he play? Martin Bailey, a pupil in a school with a drugs problem.
Where do you know him from? Coronation Street.
5. Catherine Tate.
When did she first appear? Season 9, Episode 99: "Give and Take", in 1993.
Who did she play? WDC Palmer, a stern police officer investigating a drugs offence.
Where do you know her from? The Catherine Tate Show.
6. Paul O'Grady.
When did he first appear? Season 3, Episode 33: "Personal Imports", in 1988.
Who did he play? Paul Savage, a drag artiste who is running rent-boys in the area.
Where do you know him from? The Paul O'Grady Show.
7. Ray Winstone.
When did he first appear? Season 7, Episode 85: "Innocence", in 1991.
Who did he play? Simon Fielder, who is interviewed about two boys having been reported missing.
Where do you know him from? The Departed.
8. James McAvoy.
When did he first appear? Season 13, Episode 75: "Rent", in 1997.
Who did he play? Gavin Donald, a 15-year-old Scottish boy who is interviewed about the hit-and-run of a young busker.
Where do you know him from? X Men.
9. Sean Bean.
When did he first appear? Season 1, Episode 4: "Long Odds", in 1984.
Who did he play? Horace Clark, who holds up a shopkeeper at gunpoint.
Where do you know him from? Lord of the Rings.
10. David Tennant.
When did he first appear? Season 11, Episode 128: "Deadline", in 1995.
Who did he play? Stephen Clemens, a violent paedophile who abducts and murders schoolgirls.
Where do you know him from? Doctor Who.
11. Roger Daltrey.
When did he first appear? Season 15, Episode 45: "Cracked Up", in 1999.
Who did he play? Larry Moore, who speaks to an ex-con who claims to have gone straight.
Where do you know him from? The Who.
12. Ralf Little.
When did he first appear? Season 17, Episode 76: "Night Games", in 2001.
Who did he play? Tommy Lawson, a hotel rep who accepts a bribe to set up two cops.
Where do you know him from? Two Pints of Lager and a Packet of Crisps.
13. Lorraine Kelly.
When did she first appear? At some point in 2003.
Who did she play? Herself! A cheeky guest appearance!
Where do you know her from? GMTV.
14. David Walliams.
When did he first appear? Season 18, Episode 45: "034", in 2002.
Who did he play? Ben Fletcher, an animal rights activist who takes part in an anti-vivisection march.
Where do you know him from? Little Britain.
15. Emma Bunton.
When did she first appear? Season 9, Episode 22: "Missionary Work", in 1993.
Who did she play? Janice, who just leans against a wall in a youth club.
Where do you know her from? The Spice Girls!