Thank you for helping out, Joachim if you could measure yours for me I'd be most appreciative.
I'll keep an eye out for the Bitubo shock, but I'm sure I've read it somewhere that they aren't a straight drop-in, but worth keeping in mind.
I'm thinking if I can match the GFR close enough, then I can trawl tinternet for a 2nd hand shock for the donor bike which I can hopefully get rebuilt at around half the price of new, failing that I'll have to bite the bullet and pay out for spanker. A brand new shock was something I was hoping to do further down the line once it's all proven to be working fine. I might even go the bespoke unit route at that point in time. Thank you all for your support and suggestions, I'll make contact with Robin at Falcon Shocks to see what he can suggest.