When 'Extras' returned for its second season, over a year had passed since Andy Millman (Ricky Gervais) had sold the idea for his sitcom to the BBC. So even though the second season premiere aired in September of 2006 and dealt with the events leading up to the premiere of the show within a show, "When The Whistle Blows", it was showing us what had already taken place in Toobworld. (And probably at least half a year or more before the 'Extras' episode aired.)
If so, the catch-phrase "Are you havin' a laugh?" was probably already becoming part of the pop culture scene in England. This could be why Linda McIntyre used it while talking to Detective Gerry Standing in May of 2006 ('New Tricks' - "Bank Robbery").
Or it could just be a coincidence, and that phrase was more common than I expected; that Andy Millman was just cashing in on something already in public use.
What do I know?