Jimbo - I can eat twice as much as you and still be just over 10 stone. Lol, it's strange how some people seem to get so much more out of what they eat etc. I think for me the reason I don't gain weight could be down to my lifestyle, and possibly stress/worry...I've always been a worrier! Everyone I know who can put on weight like you can is as laid back as can be.
Tips wise...definetly cardio to get rid of the fat you don't want. To be honest, with a good diet and a bit of weight training you could no doubt pile the muscle on.