It's not something you want to take a chance on really. For the sake of £500 (max) for the new cambelt, metal water pump and all the other bits + fitting, it's well worth doing.
I know it's money you'd rather be spending on more interesting things... but if it does fail you'll regret it.
Also, don't rely on mileage alone as an indication of when this should be done. Mileage tells you how far the car has travelled, but doesn't tell you how long the engine has been running for! Just think about all the times you've (or whoever had the car before you) have sat in traffic with the engine running... the belt is still working away then! It just doesn't show on the mileage, that's all. That's why it's important to get it changed based on time alone if you're mileage per year is below average.