Thanks Jackthom.
I have programmed the HW to start 5 minutes before the heating; sometimes the heating will then continue after the HW tank is satisfied. Today it shut off a few minutes later and then seems to come on for a few minutes every hour or so. Yesterday it worked OK.
I have DHWC system with separate 2 port motorised valves for HW and Heating. The HW circuit works satisfactorily but, from time to time, the heating circuit will only work for a few minutes at a time and then shuts off. Is this a duff motorised valve?