"I would say GPS with 1:25K mapping is overkill, but if you like gadgets then go for it and tell me how wonderful it is!"

I did say "what I'd really like" and I accept that I'll probably have to settle for a lesser solution. 1:50K should still be pretty good.

"Just out of interest, which PDA offers enough storage to have 1:25K maps on them? or is it a case of just putting relevent maps on the unit before you go out?"

I envisaged having the maps on my laptop and download the PDA with the area I'm currently visiting. I know it's a bit cheeky, but I reckon I could write some code to rip the appropriate mapping from streetmap or multimap and turn it into mapping data for the PDA. If this code ran on a PDA that has a built-in phone the mapping could (in theory at least) be updated from a live connection as soon as the GPS indicates travel out of the area that's currently loaded!

"Basically, which PDA/BT combination are you thinking of?"

I haven't gone into too much detail on this yet, but Navman with HP or Compaq seemed a possible combination.