Worcester Bosch CDI35 fault?

When the hot tap is turned on first thing in the morning only cold water comes out, so we need to turn the tap on and off numberous times and if that does not work we switch the central heating off. Then the hot water stays on all day no matter hot long we leave it. Sometimes when filling a bath the hot water goes off for 5 or ten seconds and then back on to hot again.

Any Ideas?

Thanks