From your description it would appear that your thermostat control valve is a little faulty for when an excessive temperature is attained the ECO device will register and overheat situation and shut down both the main and pilot burners – the pilot cannot be relight until the temperature drops to below that set as the overheat by the thermostat. Once you have lit the pilot again and the main burner lights it will continue to run until it again reaches an overheat situation. You can overcome the problem by lowering the temperature to a little below that of maximum – but I would suggest that you concat your supplier and get a gasfitter to come and replace the thermostat and set the pressures etc up as they should be.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians