Wow, thanks for the quick reply.
Well I've had the barrel in and out like nobody's business but I can't see anything obvious wrong with the hop up unit. I'll try and get some pictures soon, my phone's on charge at the moment.
Edit: I got a couple of pics of it. As I said before I really can't see anything obvious wrong with it. =/
http://img154.imageshack.us/img154/8...pbottomzi6.jpg
From the bottom.
http://img241.imageshack.us/img241/8018/hoptopjs5.jpg
From the top (the silver thing is the >>>Hop>>> label)
Edit: Anyone?
Edit 2: Never mind, I think I know what's wrong now. There's a piece of plastic at the front of the unit that's snapped off. God knows where it's gone to but meh, a new metal unit is only about £25/$52.