All of Heatwell thermostats have been cleverly designed in 3 parts.  The Thermostat, its wall mounting plate, and the electrical block.
 
An electrical surge can blow underfloor heating thermostats.  Replace the electrical block without having to replace the whole thermostat!  Spare or replacement electrical blocks can be purchased individually.
 
Electrical Blocks are only applicable to Heatwell thermostats from HT2 onwards