My mate runs an MOT place in Glasgow and said broken springs are one of his most common failures. He reckons he will many broken springs every week.
His thoughts are that it's a mixture of stiffer suspension with lower profile tyres being more popular, thinner springs to save weight (and money from the manufacturers) and certainly up here, absolutely dreadful roads. I always know I'm near home after my summer holiday in Cornwall as I notice how much worse the roads are up here.
Most are not catastrophic failures, but breaks near the end of the spring where the spring diameter is smaller, so the car/van barely sits any lower and doesn't knock etc so the MOT is the first time they get spotted.
My 10 plate Caravelle (std suspension) had 2 broken rear springs a few years ago that I didn't notice for the reason above, until the MOT.
They maybe had 5 inches missing and they were sitting almost normally. If it's a front strut, he often finds the broken bit of spring in the cup.