no hot water or heating from vokera excell sp80?

over the last few days the taps have take along time to get hot. Now we have lost all hot water and heating. Have checked the boiler and the fan & pump are running but the fan stops when you call for hot water. Have turned the mode selector to off and then back on again and the boiler fires up but a clicking noise is coming from the pcb. It clicks for about 20 seconds and then cuts out again. What do you think needs replacing.