Thanks for the offer Ragman. In the end, I figured for £65, I'd be as well getting a new amp and having a spare if that wasn't the problem.
Having fitted it, the amp clearly wasn't the problem.
This can only leave the TV tuner module as the source of the problem. I had assumed this wouldn't be the problem, as the TV picture and satnav display all works fine up front, but I really can't see what else it could be. I don't
think it's a loom problem, because it gives a little bit of sound when the radio is first plugged in, and then dies off, which certainly doesn't sound like any wiring problem I can think of - more like a faulty electronic component inside, I'd think.
If it is that, it's going to be ****** annoying! Pay £100 or so for a soon to be largely obsolete part, or £450+ for a digital/analogue hybrid unit!