Asda are selling these Sat Navs for 50 quid in store. Don't know much about them just yet but will add some more details later. Uses Teleatlas (Tomtom) data and has a 3.5" screen.
Most importantly runs WinCE 5.0 so hackable for Tomtom (up to version 7 now I believe on CE 5.0).
Getting my hands on one tommorrow so will update of progress of hack. I think the hack should be similar to the ones from ebuyer or this one i did on another cheap Sat Nav from Asda:http://www.pocketgpsworld.com/modules.php?name=Forums&file=viewtopic&t=48563&postdays=0&postorder=asc&start=30