I started with a new company and while playing with one of the mains controllers they supply, I used my heater as a test load.
Two problems with that.
First the heater on half load is about 500 watts and the mains controller is only really supposed to be used with loads up to 300 watts.
Second me! I could and should have operated just the fan section of the heater safely in the limits of mains controller. However I forgot and at some point in my testing I switched the heating section on.
This resulted in a heater having enough power to get hot but not enough power to drive the fan properly.
Had I not been full of cold I may have smelt the heater before it tripped the not reset-able thermal fuse.
My wife smelt the issue and asked me if I had been letting the smoke out again! It was too late by the time I investigated the equipment.
Today I have taken the unit apart and confirmed the RY 121 Thermal fuse is open circuit. It is rated 250V 10A 121 degree Centegrade.
I have ordered 5 for 99p off a auction site.