Range Truibune - Central heating ok but no hot water

[Nerdblot]

Help pls. I have a Range Tribune unvented system - indirectly heated.
Over the summer I have the central heating switched off at the timer, but leave the timer on for hot water. The hot water has now become unpredictable, sometimes it fires up, sometimes it doesn’t.
However when I turn the central heating back on at the timer the hot water starts heating normally & will then work OK for a few days.
Because of this I am presuming the fault is not a timer fault, but a fault at the hot water cylinder. I note it has a dual thermostat. Could this be the problem & how would I find out/sort the problem?
Thank you.