As KillerOnion said, it's very likely to be the reg & it is very easy to clean. One tip on how to get the large and small piston out after removing the circ clip is to hold the reg open end down and lightly and rapidly tap the reg onto a hard wodden surface until they drop free - think woodpeckers & you get the idea, rather than driving rail spikes with a sledgehammer. Be careful not too lose any parts though, especially the $%&!" ball bearing. Clean all the parts, oil/grease and reassemble, making sure you get the spring pack in the right way.
It's a good idea if you can check on a different air system as well, even though yours is a preset, it could have a fault developing.
A very easy thing to check is to see if the bolt is dragging: just see if it's sliding easily within the breech block and whether the anti-double ball is depressing easily; if not, then clean the bolt, breech, anti-DB as appropriate. You say the LPR has been checked so that shouldn't be the problem - although there's more than them being in the right place??!!. It could be the ram/snap ring, but you really should get the tool kit for this as Mark said, although they can be done without one, but I don't recommend it.
Angels really are simple and maintaining the mini-reg, LPR, snap ring & trigger pull shouldn't be beyond anyone & can save you a lot of time & heartache. Buy a tool kit now, you won't regret it! (whispers out of the corner of my mouth to Frazer: "Was that OK, Fraz? Or should I have sold him an AIR as well?")
