Potterton Prima F50 startup problem?

initially the boiler doesn't fire up when instructed to by the timer. You can hear what sound's like a fan whirring then it stops and trys again. This can happen for a number of times until there's finally a click and the boiler fires up and the flame is ignited. Today though, it did'nt start for over 3 hours!

This happens at startup if it's for the central heating or just water heating. However, if it's on the central heating, once it's finally fired up everything is ok.

If it's just heating the water the then fault appears again every time it want's to ignite the gas. I've had an engineer have a look but he couldn't find anything and I'm tearing my hair out with it!