Adrian wrote:

[...] I'm only guessing that it is a thermal fuse but it might be
some sort of regulator, it is brown rectangular block with a ceramic /
white interior and has 3k8-J then greek resistance symbol and then the
ohm symbol. Underneath it has 113-102, but no other marks.

That sounds _awfully_ like a resistor to me: 3800 ohms and "J" means
+/-5% tolerance. Are you sure the value isn't 3k9, or 6k8, as these
would be more 'standard'?

What it's doing in a storage heater is a good question - accelerator
resistor for a thermostat perhaps.

I can't help much more I'm afraid, have you tried phoning Glen-Dimplex
to see if you can find a helpful service bod?

--
Andy