>>> I had a service last year
Yes, but what about your heating system? Did that get one too? ;-)
[ Sorry, I just couldn't resist that! ]
For a more practical answer from me though . . .
Under 'How to diagnose a faulty diverter valve' here
https://www.superwarm.co.uk/ultimate-guide-to-diagnosing-a-faulty-boiler-diverter-valve
it states
"If you are getting all the hot water you need, but no heating, first check all your time clocks and controls are working. If everything else is fine, it’s almost definitely the case that the valve is not opening for the heating"