Hi Everyone
I've finally decided what to go for. Fortuna bluetooth GPS with HP iPaq 2210 PDA, Navman and Memory-Map software. This gives me 1:50k mapping available for the whole of the UK for walking. The software can also load scanned (or downloaded) maps. The 1:25k is only available for certain areas and is horrendously expensive (e.g. Lake District = £99), so I won't be bothering with that until the price comes down substantially.
I am assured that the bluetooth GPS unit will operate in a rucksack or pocket and there is an hermetically waterproof neck-strap-mounted case that has a flexible transparent front so the PDA can be operated while inside. It's also designed to float with the PDA inside it.
The kit will also function as an in-car navigation system with street-level mapping for the whole of Europe and on the basis of this I can justify it as a legitimate business expense. OK, it's a little more than just a tad expensive, but I was planning to buy a PDA anyway and the bluetooth GPS (including the Navman software) is only slightly more than a mid-range hand-held non-mapping GPS. By the time I get the VAT back it will have cost me almost nothing. ;o)
I can get flash memory cards at trade prices (e.g. 256MB CF £27), so the maps can be left permanently loaded on a card and just swapped in as required. If anyone else would like cards at these prices, let me know.
Thanks to everyone who contributed suggestions. I'm hoping to have the kit by Thursday in time for Silbury and the West Country trip.