Hi Keith there is an American website completely devoted to boolit casting, if the answer isn't in here somewhere I will be very surprised, as said before casting muzzle loading bullets needs soft lead, old airgun pellets and church roof is good, does your club have a butts digout once in a while? good source of casting lead.
Try this link, hope it works, have fun, don't breath the fumes and keep everything DRY, beware the "tinsel fairy"!!
http://castboolits.gunloads.com/

http://castboolits.gunloads.com/forumdisplay.php?f=8

All the best with your casting; John