So.... 5 weeks on we have a result. Supplying dealer washed their hands of it, then quickly backtracked under pressure from Vauxhall, finally agreeing to pay 10% of costs. They wouldn't recover the vehicle though, so VX kindly delivered it to them and returned it at their cost, in addition to covering the other 90% of the £10100 cost involved. We now have the car returned with new transfer box, propshaft, and rear differential! Technicians think the u-joint on the prop went, forcing it out of alignment and resulting in damage to the adjoining parts. It was a long uphill struggle but so pleased we stuck at it with encouragement from all who commented here, so thank you all.