Try Christchurch gun club (www.christchurchgunclub.org) they have an active black powder section.
The difference between black powder and pyrodex is one is an explosive (black powder you need a licence to posess and approved storage) the other pyrodex is not classed as an explosive and it can be bought without a licence. Both are as corrosive as hell when fired and are messy to clean up after.
777 is made by Hodgen who also make pyrodex, it is a little hotter than black powder and a lot less corrosive also:mad you need to reduce the load if using instead of Black powder. It is also a lot easier to clean the guns after use.