Sorry… re costs…
If the crown wheel/pinion are not badly worn then you’ll be looking at a pub lunch over £2k to get Steve to overhaul your diff, replace the bearings and seals before fitting a TracSport and shipping it back to you.
Ask me how I know… 🤔😳
If the diff is trashed (or you want to start from scratch to give peace of mind), add circa £1500 to that for a new open diff from Caterham.
Getting the diff out is comparatively straight forward (assuming you don’t smash your finger trying to undo a bolt, otherwise it’ll be an elapsed time of over a week; again, ask me how I know). Getting it back in and centred is much more of a ‘mare however.