Expansys are flogging the Nokia NavKit for the 770 / N800 here:

http://www.expansys.com/p.aspx?i=151567

for 25 UKP + postage. But, if you go to hotukdeals.com & search for expansys, you'll find a simple way to grab a 10 UKP off voucher via another forum.

So you can pick up a Nokia LD-3W bluetooth GPS, N800 car charger & mount and a 3 year licence for the Wayfinder SatNav for 20 quid or thereabouts.

I think that's a pretty good deal all round personally.