First try, on a local but rather convoluted route, was fairly successful - once I'd found the route buried in the Apps section of the Nuvi
Have now created a longer route, which I've uploaded and will try over the weekend.
These routes have been created on Basecamp, which so far seems OK for car routeing, but my next aim will be to try and upload routes created on Cycletravel, which is more cycle-friendly. My Cycletravel routes, incidentally, contain hardly any off-highway stuff, so shouldn't be too much of a problem - I hope!
Thanks again for the advice.