I have one 7 litre DIN fit cylinder here, in good condition, with almost 2 years test on it, which I'd be prepared to sell if it helps. It's a standard cylinder, so easy to get one other to go with it.
I'm pretty sure a certain person in the CDG Northern Section would get it to Sheffield or Castleton for you.
PM me if you want to follow this up. No worries if not as I'll just keep using it.
(I might also be able to find you a second 7 litre cylinder in decent condition to go with it but this 2nd one would need testing, so cheap. Need to check this . . . if you're interested.)
I also have a steel backplate / harness here going spare (very cheap, as this is just for donations to a certain good cause).