Well, I owned a Sniper II before. The only difference between the two is that the Sniper III doesn't have a vertical ASA adapter and is a little bit shorter.
Anwyays, the Sniper series of guns are quiet and accurate.
The gun can be heavier than other pump guns.
The cocking/pumping action is smooth.
If you get bored like me, you can change it into a Mini-cocker.
My only grief: it doesn't have a slam fire/auto-fire trigger like other nelson based pump guns (like the Sterling, which I also own one of!) But that's OK. You don't play pump for the ROF!
I don't know about where to buy on in the UK as I live in the US. Sorry.
bouncer