I get my 2kg bottle refilled from Ocean Commotion in Leicester. It has a weeks turn around (sent away on a tuesday and arrives back the following tuesday) and it costs around £16 for a 2kg bottle. I beleive a 500grm bottle will be £12.

Tom, my experience with local welding supplies is that they want to rent you the bottle for a year before they refill which is a real pain unlesss you want a 20kg bottle or something like that.