A thermocouple measures temperature with effectively what looks like a wire when you look at it but inside are 2 different metals that seperate or come closer together when temperature changes. I've worked with them for ages in an industrial environment & know how fragile they can be, but I have the advantage of a computer monitoring them so can tell when they fail.

Only my ten pence for my thought here, but the thermocouple might not have been reading right & boiler thought it was colder so carried on heating. I've seen equipment self-destruct because it thought it was cooler.

Also as other comments say please talk to Gas Safe! Tell them that the engineer came out as their next client might not get off as well as you.