johnson & starley model J25 - 32 Mk3 modair flow water heater thermostat?

johnson & starley model J25 - 32 Mk3 modair flow water heater?
Hi I have the above model warm air heating system, however I have been informed that the thermostat (which is mercury) is faulty and you can not buy the individule part is this true?

Is the termostat part of the gas valve?

I have been told i need to buy a whole new water assembley unit 400 pounds plus but replacing the whole unit just for a thermostat seems crazy is this the only way?

Thanks
Steve