Been out to an old Worcester 350 RSF boiler. Was told that it wasn't working on ch or dhw. When I arrived the ch was working, but was then told that the dhw wasn't. Opened a hot tap and the boiler fired up and the water ran nice and hot for about 3 or 4 minutes. Turned tap off and shower on and boiler didn't fire up. Tried hot taps again and still no hot water, but green "Demand" light lit up. Called technical help line and was told that all the checks I conducted pointed towards the main driver board requiuring replacing. Thermister reading as it should be, but power to the fan (Pin X17?) giving a reading of less than 1v. I'm a bit reluctant to change without a second opinion, due to price of part! Any ideas?