Why does the thermal cut out keep switching off my Dimplex XT18?

I have a Dimplex XT18. The heater input is usually set on 4 but most nights the thermal cutout switches off the heater after being on for a few hours. There is a wire that connects the thermal cut out to the charge controller and the sheathing on this wire has corroded away (due to heat). The thermal cut out is triggered by this bare wire arcing across to the storage heater casing. What spare part do you think I need? I was thinking of just replacing the wire/sheathing, but why would it get so hot in the first place?

Geoff Cripps
January 2008