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(mercury) and you can not by the individule parts 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 punds plus just for the part is this the only way?

Thanks
Steve