Baxi 105e HW but no CH?

Hi
Ive got a Baxi 105e which has developed a CH fault. Hot water works fine but flame will not remain lit when set to CH. It just constantly fires up then dies down, then the flame failure LED flashes, and it requires reseting. Had Engineer out who changed pcb and flame sensor (which works fine on hot water), but still same problem. Checked voltage supply, found to be 216V. Engineer thinks fault is due to low voltage. Called electricity supplier. They measured the voltage at the meter and concluded that it was within their voltage tolerance of between 216 - 253V! Phoned Baxi, who weren't very helpful, they couldnt give a tolerence for the supply voltage the boiler required. "Needs 230V mate, thats all I know". So now Im stuck in catch 22 with a boiler that doesnt work and a low supply voltage.
Am going to hire a generator/ invertor to put 230V supply on boiler to sort out whether its the supply voltage or the boiler at fault.