I would send it to someone who knows what they are doing. There must be a local specialist in Leicester? When I got my LSD and final drive installed by a specialist I was charged £200.
Personally I think you were on the right track beforehand.
It's just a gearbox, nothing special about it in the scheme of gearboxes just cause it was/will be attached to an h22.
The mfactory diff is the special bit and a proper gearbox builder will have dealt with more of them than paw will have, so they are actually the relevant specialist here imo.
Well going off mfactory specialise in Honda and Mitsubishi and paw are Honda/mitsubishi specialists. Plus iv used them a couple of time and there Facebook pages are full of gearbox rebuilding porn.