Right then, I reckon this mystery's solved (or solved enough for me).
Talked to the company who took over support for the radio manufacturer last night. It's apparently a custom made connector (!) and the cables are £50 each, plus postage, plus VAT, with a several week lead time!
So in the true spirit of bodgery, I stripped the thing to bits and replaced the connector with a GX12-2 "aviation connector". Added fresh cable (mmm, silicone wire!) and a PowerPole and now I have a working Mobitex radio to play with!