PDA Memory Map & TomTom GPS System
I have been using an HP Ipaq 1710 system for both TomTom (in the car) and MemoryMap (in the air). As others have said, not to be used as primary navigation, but a good backup for a position fix.
I've since upgraded to the Garmin 496, but it was a good stepping stone for me. If a person were to be interested in the PDA GPS option, they could maybe find a 1p starting bid on such a system, on a popular internet auction site. Search for: Ipaq PDA & GPS TomTom & CAA Memory Map England South
Merry Christmas!