Why doesn't my worcester 350 give hot water?

I have a worcester 350 combi boiler and programmer. It recently decided to stop delivering hot water. When a hot tap is turned on, I occasionally get a few seconds of warm water then it goes cold. The standby light stays on but the demand light doesn't come on and nothing appears to fire up.

The central heating and other controls all work ok.

I followed the fault finding flow chart in the manual and it looked like the main driver board or flow switch was at fault. I then got the main driver board replaced but had the same problem. Is it likely to be the flow switch or should I look elsewhere for the fault?