Hi Tony,
I think so.
If I go back to basics
The boiler is on waiting to go into action
If I have just the heating on and the thermostat down - nothing happens, if I turn it up, the boiler bursts into life and heats the radiators
If I have just the water on, I can do the same by changing the thermostat on the cylinder
So when both are required, sometimes it works, sometimes only the water side works. I did think that the cold may have something to do with it, but we noticed it first in the mornings, when both needed, but no central heating came on
Do I need to know any more?