Protherm parts - where to buy overheat thermostat?

Protherm 80E giving no gas and no ignition and F1 fault, fiddled with the reset on the overheat thermostat after trying everything else and it works again. Been an intermittant problem that suddently got worse (presume thermostat is tripping too early and now getting stuck and not resetting when cooled).

But I'm having great trouble finding a replacement, it should only cost about £13 but where! Please help!

Greg.
direct email greg AT gregandsarah.co.uk if anyone wishes. Cheers.