OK, so I've turned to the Dark Side and got myself a RadarBox. Only minor snags apart from the hardware disconnection problem. I've sent an error report direct to AirNav Support, but this message is just to keep people informed.
I'm running an AMD 64 Athlon processor based system running at 2Ghz and with 1 Gb of RAM. Operating system is Win XP SP2.
I've tried the obvious things, different USB leads direct to 2 different USB ports on the PC. With and without network data. Disabling other USB hubs/devices - but I've not been able to solve the problem yet.
Funnily enough, I haven't suffered the disconnect when using my laptop (similar specification, but newer that the desktop) - although I've not had it running on the laptop for any length of time. I do get occasional USB disconnection problems with the SBS-1 as well, but these are exceptional rather than routine. It does seem like we have a trend here with the RadarBox software, rather than a problem with individual PC's.