how can i fix my worcester bosch 24i rsf boiler?

in need of some help with my boiler please, i cant get ch to work unless i turn the hot tap on to let the boiler fire up and then switch the ch on. if i try to put ch on without the hot tap turned on the fan runs and pump but doesnt spark and ignite. also if the programmer and thermostat are calling for heat for the ch and the boiler doesnt ignite i cant get any hot water then until i stop the the programmer and thermostat calling for heat.to make things sound more confusing i have learnt that if the ch is on then is turned off then back on again it seems to be fine,but if i turn the hot water tap on in between the ch being off the ch wont come back on (fan and pump run etc but no ignition). i know this because the ch is set to come on at 5am and go off at 9am which it does, then back on again at 4pm and off at 11pm which it doesn't, i think this is because the ch goes off at 11pm and after this no one in the house turns a hot tap on,but during the day the hot tap is on and off all the time.seems the hot water supply interupts the ch in some way. i have heard an read could be the flow switch or thermistors but could do with a bit more advice.by the way the power light isn't flashing which i am led to believe would be a sensor fault.