Likely cause of boiler transformer failure?

I have an Ideal Icos HE15 heat-only boiler which, judging by the archives here, I understand to be the unwanted child of the boiler world. That said, this one is 12 years old now and has given sterling performance for the 9 years since we adopted it.

Until now that is. Today the house was cold and the boiler was dead. Many years ago I'd bought a spare PCB as I was already aware these were its Achilles heal and, sure enough, replacing the PCB has brought it back to life.

First thing I noticed with the failed PCB was that the plastic case had a small area that was dark and slightly warped on the outside:







Opening up the case revealed that the board looked quite healthy however the transformer, which is situated right beneath where the warped plastic is, was a little dark and the label showed signs of what I guess could be heat stress:





Measuring the primary coil revealed it was open circuit.

Any ideas what could cause the transformer to fail? Do I dare rejoice that I've fixed the boiler, only to find out soon (Christmas Day probably) that there is an external component (e.g. fan?) that might be overloading the transformer causing it to fail? Or could it just be age / poor design?

Incidentally, my unhealthy interest in this transformer is down to being keen to work out not just what failed but, more to the point, why.

Are you gas safe registered. You have taken a gas critical safety device out your boiler. do you know the correct 26/9 checks to carry out if you replace this.
you fitted a new pcb but should get a registered installer to fit it as you don’t know if you did it ok.