I forgot to update this but basically:
It was the control board at fault.
Turns out that these boards contain three problematic capacitors which, over time, start playing up. Replacing them (for next to nothing) fixed the fault perfectly and it's run well ever since.
It was the control board at fault.
Turns out that these boards contain three problematic capacitors which, over time, start playing up. Replacing them (for next to nothing) fixed the fault perfectly and it's run well ever since.