worcester 24i rsf. intermittent pump and unreliable burner firing?

problem with boiler - when i demand hot water, the pump will work but sometimes burner will not fire up, sometimes it does and then water goes cold. Central heating will sometimes kick in on timer, sometimes not. most curious though is the pump which turns on and off intermittently for no reason. I hope someone can help ! I have tried the manual fault finder/diagnostic but am unable to identify the pins and their respective numbers to go any further. .