You lot have jinxed my sister... She phoned me to say that a boiler in one of her rental properties has 'popped', spraying water all over its insides and blowing its circuit boards. She checked and it is 6 months out of warranty. She has been quoted £1,200 to have it fixed!
And yes - she does have it gas safety checked and 'serviced' every year, the last time being a couple of months ago. (I'd be asking why the engineer didn't spot the impending failure, but maybe there
isn't any sign until (say) the pipe splits.)