Entry-level heart rate monitors used to cost about £60 a few years ago. Now you can pick them up for half that. Most budget HRMs, like Polar's F1, work fine but have few functions. This Halfords HRM has plenty.
They are: current heart rate; average heart rate; maximum heart rate; settable target heart rate zones (with an alarm if you go outside the zone); exercise time counter; time spent in target zone; time spent over target zone; clock; stopwatch; and calories expended (which I'd take with a pinch of salt). There's a backlight, it's water resistant, and it comes with handlebar mount if you don't want to wear it on your wrist.
It didn't interfere with my computer's wireless signal and the readings seemed accurate. Operation is no more complicated than most bike computers, and the two-tier display (e.g. heart rate and stopwatch) is easy to read. If you're serious about improving your Personal Bests, a cheap HRM like this should prove a better investment than a computer. If your heart rate is on target, you're going as fast as you sustainably can; and if it ain't, you ain't.