ravenheat csi85t combi boiler ?

when i turn the tap onthe boiler comes on then the ignition fires once the burner lights & instantly shuts down, it will continue to attempt ignition & will occasionly stay on untill the tap is turned off or it will shut off after about 2 minutes, if the heating is on when the tap is turned on the dhw. is usualy ok ,boosting the burner as it should . at the same time i am having a problem with the heating coming on at random when it is not set to come on & i have to switch it of at the isolator. i would appreciate your thoughts.
thanks geoff.