Just in case anyone's interested or encounters a similar problem, my GPS receiver is now back up and running again. I took the battery out of the unit for 48 hours and then put it back in and fired the thing up. I left it switched on in the garden for 30 minutes doing its "so where am I" thing and then linked the PDA to it (bluetooth) and bingo! Perfect.
Why do electric/electronic things have to contain so much magic and give the outward impression of being perfectly all right (emissions of smoke and light excluded) when they're not?
Steve