Thanks for the offer from Sheffield , if I am heading that way I will give you a shout.
I have tried civic amenity tips and scrapyards but both say they can no longer sell gas bottles.
whilst it's 3 or 4 years since I last did it, our local tips were happy to let you take a cylinder if you ask nicely and aren't too official; the cyls may be marked "property of Calor" etc, but you are going to take it back to them for a re-fill, aren't you? Just helping the tip guys do their recycling a bit easier etc...
You can always get a slightly larger one and swap it for a smaller one, or swap butane for propane, esp at a proper dealer rather than a camp site.
I found that if you don't make a fuss. Go at a quiet time. Just park next to the bottle cage with a fiver in your hand. You may find its half full of gas. Serious bargain!
Richard