Your problem lies with the main gas control valve that has a built in thermostat – the fault is the “differential cut in/cut out” setting of the thermostat. That is in simple terms the number of degrees from initial switch off at set temperature until the drop temperature is reached causing the thermostat to cut in and light up the burner to reheat again.
This is a common enuff problem and can be easily overcome by changing the main gas valve assembly – not nesessarily with one supplied by the heater supplier.
It is however, most certainly a job for a qualified tradesperson to undertake as the gas pipework has to be disconnected and reconnected, pressure settings undertaken etc, so don’t try it yourself!!
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians