Water only heats when heating is also swithced on

[jazz-uk]

I have a traditional (ie not combi) water and central heating system run by a 20 y/o Chaffoteaux & Maury boiler.
Problem is if water only is switched on, the pump starts but the boiler doesn't heat the water. You can hear the water pump through the boiler but the boiler doesn't flame up.
If heating only is on, house heats up nicely. If both are on, then I get both hot water and heating.
obviously its not a worry while we still need heating, but when it warms up, how will I be able to get hot water only.

 

[MANDATE]

It looks to me that you have 3 position diverter valve stuck at the mid position giving both HW and CH but will not go to HW only.
You should be able to remove the motor head and turn the spindle easily by hand. It may be the motor needs replacing.
If you search past postings on this subject there is lots of information.