Boiler has died

WIAWSNB

Britannia12345 said:

The fused socket has a wire to the boiler, then a wire from the boiler to the thermostat. another wire goes from the fused socket to the alarm, and a smaller wire runs off that to the alarm controller. there is no backup battery in the alarm - once the power is turned off it doesn't work. 

Thank you.
That would appear pretty conclusive, then.
The FSU is working fine, as the alarm is still receiving mains power.
The thermostat is supplied by the boiler, so the electrical 'break' is almost certainly within the boiler itself.
I understand there is a PCB-mounted fuse, but beyond most layfolks' remit to check it, I'd suggest. Worst case would be as said by others - a bust PCB.
Repaired boards can be had for around £35, but you'll need a sparky/plumber happy to fit one, and it's unlikely they'll warrant the job. However, repaired PCBs should go on to last the remaining life of the boiler, so are a sensible option. 

Britannia12345

Yep, the pcb had gone. Having a new one fitted right now.