Thanks for the help (yet again!).
When I took the wee box off the boot lid, it did seem a liitle damp - but no amount of appication of the hair dryer improved the 'lost' sat nav.
So, then I went rambling thorough the sat nav menus and eventually found a section where you can manually set your location. After a couple of 'careless' atempts, I managed to let the sat nav know where it was and, I think, got it all sorted.
In hindsight, I wonder if this was a result of a serious battery failure (new battery since installed) and the system simply needed to be re-set?