In reply to GW7AAU:
I have the UK topo maps on my Vista and I think they are excellent. Of course they are no substitute for a real map, and the small screen is very restrictive, but nevertheless it does make it a lot easier to visualise where you are. Mapping of paths is a bit sporadic, but enough are marked to be useful, and being able to see your path crossing the contours gives a good indication of progress (sometimes depressing when you realise how much is still to be climbed).
It is worth mentioning that the UK topo maps also have pretty good street-level mapping in towns and cities. This aspect isn’t very strongly promoted, but it’s all there and is very useful when you do have to navigate around towns.
I have a 1GB memory card in my GPS and that is enough to hold the maps for anywhere I’m likely to go in the UK, so I don’t need to worry about swapping maps in and out. 2GB cards are now available at reasonable prices and one of these would hold the entire UK set.
Curiously, the Isle of Man is omitted even though it is mapped by the OS. None of Ireland is included, but that is understandable as the raw data is held by a different organisation.