I was hunting around for it since buying it for my O/H a few months ago, seems like an excellent game and very funny.. Cheapest I've seen it...
Also going through Flubit took me down to £17.87
(didn't know if this was a solid offer so didn't put it as main price)
You have to click the sellers to find the guy selling at £21 & free P&P their seller ID is "gsm m"