Welcome to a truly addicting facet of the shooting world; there's nothing quite like double rifles!

Your comment: "Got loads of ffg to shoot up!"
You will find with the Black Powder Express doubles that acceptably accurate loads won't be quite as easy as just using a measured charge of FFG from Elephant or Goex. Simply not the same quality and power as the original C & H #6 that was used.
I would not be the only one to recommend that you get a copy of Graeme Wright's book "Shooting the British Double Rifle" if you want to achieve early(er) success with accurate double shooting.

.50 BPE is great fun and a solid hunting caliber, but don't discount the .450BPE either.....especially if you are hunting deer size game. Bullets are somewhat easier to find and if you are at all recoil sensitive then the .450 Express is a bit easier.
Actually, it seems like a .50BPE is more commonly found for sale than a .450 just due to the popularity of .45 caliber.

Look here for a feel of availability and prices:
http://www.gunsinternational.com/English...amp;start_row=1

$10,000 - $14,000 for a good solid clean name English BPE in .450 or .500 is about standard these days.

- Mike