Garmin forerunner, the 110 is just over £100 on amazon (may be cheaper elsewhere).
The GPS ones have three-digit numbers, but you may be happy with the cheaper footpod based ones (with two-digit model numbers) as these are more accurate than Nike + and accurate/consistent enough to help you in training.
I trained for my first marathon with a forerunner 50 foot-pod based one, and treated myself to a 405 after completing the marathon. The 405 is nicer (e.g. able to program intervals etc) but the 50 was good enough