Christ, just realised with a start I never replied here after all the helpful suggestions! What a dreadful bloke. Anyway, my lovely RR went into the specialist and it turned out the ICE/amp was incredibly corroded. There wasn’t any obvious sign of water damage in the boot but I’ll know next time to unplug the amp and inspect the connectors. ‘cos look at them:
Anyway, the ‘warranty’ the dealer promised quickly evaporated when I told him it needed a new amplifier. Thankfully he agreed to refund and collect the car. So I’m not massively out of pocket. But I do need a new Range Rover! Still looking for a post-2006 green one with cream leather...that’s been better looked after.