propane or butane?
i got a 4.5kg butane bottle from towsure for about £45 (with the gas)
I'm sure i had to take ID with me when i got it. i think it's to make sure your not a terrorist or something ridiculous, so it might be worth ringing before you go.
also try CCC's (go outdoors just of brammal lane) they tend to have very competitive prices.