you may be able to find someone who can fill it, such as a company that deals with fire extinguishers. But there's not many around. Is it a pub bottle? If so you could try your local pub.
One or two outfits supply gas with no rental on the cylinders and that's probably the best way forward, at least you know that the cylinder's been tested.
Again, there's not many around but Adams Gas is in Kent and may be usefull for you.
There's a spreadsheet about gas prices and comanies, etc, on the forum.
Other than that you're stuck with rental charges like most of us :-(
6kg Co2 cylinders rent free from these guys. Just turn up, pay 17 quid and the bottle is yours. Only requirement is that you return the empty. Pain in the bum to find the unit. Took me about 20 mins to find it! (maybe I have no sence of direction?!)