Thanks everyone for the responses! I've read them all but there were a lot of them so I'll read over them again. The good news is that I work at a small bike shop and therefore have access to lots of spare parts and can do my own work. Here's some more information on the bike.

- 105 Downtube Shifters Indexed or Friction (Absolutely a dream by the way.)
- 52/42 Crank
- 13-24 Cassette
- Non integrated BB (I enjoy taking it apart and re-greasing!)
- 105 RD Short Cage (Has "6S-7S SIS" imprinted on it.)

I believe the gear cluster on the back is technically a cassette but being of the Uniglide nature, the last cog is the lockring. It's my understanding that this was how cassettes worked at first rather than have a spare lockring that screwed into threads on the inside of the freehub body.

I have a 6S mountain bike gear cluster that would be perfect but I'm not 100% sure it's Uniglide and if it is, I'm not sure how that would work on a 7S freehub body.