Mobile Phones

[know your limitations]

So using the sat-nav for a day's journey shouldn't use much more than a meg of data then? (I realise if abroad it's a different story).

I use my HTC Desire daily as a sat nav and cover around a 1000 miles a week in my lorry. I have a 1GB monthly data allowance and wouldn't come even close to using that using sat nav alone.
Little tip for you. Once you have your route, turn off your mobile data on your phone. This will save unnecessary downloading of data (but it keeps your network signal for calls).
You will only need data on if you decide to stray hugely from the route planned or if you want constant traffic updates.
The phone downloads the map required in "tiles". Even if you make a small error following the route, so long as you don't stray too far, the sat nav will keep up and redirect you.
If you've gone too far the wrong way and off the downloaded map, just turn your data back on and it will donload the required portions for you.
On most Android phones, you can set your home screen to contain a Mobile Data On/Off toggle button to save going through the menus, so it really is very quick.