Hi mate,
Firstly, the advice the guys above have given is good. Go for a pre-charged pneumatic (PCP) rifle. Steer clear of CO2 powered rifles, because despite what some sections of the shooting press write, they aren't good enough for serious shooting (although great fun for plinking!) as their power is so inconsistent and affected by slight changes in tempreature etc.
I'm a fan of both the BSA Ultra and the Air Arms S200 as the lads have suggested. I looked at an S200 as a second rifle a few months ago, but at the moment, they're almost as expensive as the S400, so I went with an U