I dont know if this has already been listed but, If your in Northampton I use this place.
their address is,
33 Harlestone Road, Northampton NN5 7AB:
Tel: 01604 586156
I think its £5 for a 12ltr refill.
Nice guys and very "gun Freindly"
Actually are quite cheap for tanks and stuff. I think under £190 for a 7ltr tank and whip