Hi Dave, if you're using Calor gas, like most 'vanners, have a squiz in your Yellow Pages where you'll find loads of stockists, aside from C Sites & Dealers. we buy ours direct from Calor Gas who have a depot about a mile from home.
Caravan dealers and larger sites are the obvious places but other Calor dealers do exist, like hardware shops, tool hire outlets etc. If it's not a Calor cylinder, refer back to the original supplier.
Dave, beware of advice of refilling from petrol stations as overfilling of your gas bottle can have serious implications. If your bottle is overfilled and liquid gas gets into your regulator, when this liquid reaches it boiling point it will expand at up to 200 times in volume. The diaphragm in the regulator is not going to cope with this kind of expantion.
calor from their dealer network,sites and caravan dealers, but tons of probs if you are in europe on a long stay. gaz dealer network, sites some large supermarkets, sports / outdoor shops and caravan dealers however in europe anywhere.