This might be useful to someone. Found a little utility called "Tyre": -

http://www.janboersma.nl/gett/index.php

Basically it lets you plan a detailed blat route with many waypoints with google maps on your PC, then export it as a tomtom itinerary for using on tomtom device (or mobile phone with tomtom etc)

This website does the same thing.

http://www.houghi.org/tomtom/

been around for a while and I can confirm it works very nicely.

Actually, just had a fiddle with Tyre.
Nice bit of software. Much easier to use than the online converters

My old blat plan:

Pick a direction left, right, or straight on.

Only junctions that have main roads on count or you could be blatting round a housing estate.

Once you've taken the turning, repeat. Pick before you get to the junction and it's more random. That's the way I've found many lovely roads

Anyone know if there is a similar tool for a Garmin satnav?

All satnavs should have a "scenic route" or "fun route" function, like on that 207 advert