You wanna keep your eye on the shops because garmin and tomtom have just up dated all there units so the old unit should go down in price - and if you not in a hurry i would think that halfords on the next bank holiday will have a deal on.
also might be worth checking halfords out cos we were having some rac ones and some other cheap unit in (but im still on holiday so dont know what we have got). but i will warn you that there not very good.