worcester 24i rsf?

Heating comes on straight away some times.
Other times have to switch fuse on and off and it seems to work then.
Same with the timer works some time.
Is a combi boiler and hot water should run constantly but rarely works without the heating being turned on.
have had a new clcok fitted and valve for the water costing £100 but has not fixed the problem.
Has anybody else had this problem and can you offer any advise.