For anyone in Sheffield looking for a less extortionate castration, I've now found the place! Broomhill Veterinary practice quoted me around £36 a snip. They've not done the procedure before but they've done a lot of research, and are in contact with vets that do so regularly. They have a large APH clientele, so are used to dealing with exotics, but don't charge a Premium like the Ark do.
Edit: I have since used this veterinary practice on a number of occasions and the vet there has done extensive research for me, going above and beyond what I would expect so I would definitely recommend them. We have our male joey booked in for a castration there in early March
