Potterton Prima F50?

Hi guys - I have followed the common problems with these boilers (ie PCB issues) but sadly this isn't working for me - can anyone help please?! I am reasonably competent at electronics and have replaced all three troublesome caps and checked the PCB closely for other issues but all seems well. Still no starting up though. When boiler is turned on I get the fan whirring and if I turn the boiler heat control know slowly from 0 to max I can hear one relay/switch click followed by another but nothing else happens. Boiler has been working fine for the three years we have lived here and was serviced a few weeks ago but worked fine non-stop since the service up to now.