Finally got around to pulling the stereo out - looking at the little dinks here and there it's definitely been out before at some point (impossible to tell when, of course, but I'll bet right around the time the GPS packed up!).
The GPS antenna connector at the back of the unit looks fine, not mangled or mis-shapen, and the connector in the frame looks good too..
Which presumably means it's the GPS antenna itself, or the wiring to & from it - which could be a mission to track down

and I don't fancy spending a ton of time on it, really (patience for working on cars on my own wearing thin these days!)
Still.. would I be right in thinking that the wiring from the antenna goes along the rear edge of the roof lining, then down the side of the car somewhere - possibly visible from the large access panel on the nearside of the estate boot? I ask because in there I can see a non-original looking wire, insulation tape and spade connector.. and I wonder if someone has bodged an aerial loom repair at some point :-/
I suppose I could tear it all out and replace it with a double-DIN touch screen navigation thingy.. but even that sounds like a .. pain in the neck

[edit] Ooh I just noticed that according to the book of lies, the aerial has a power connector - anyone know if that's for the GPS or the mobile antenna? I'm guessing the GPS.. and maybe it's power supply has gone AWOL.
Wonder where it's power comes from.. oh for a wiring diagram!