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Hi , I have a conventional central heating system , the only way I can get hot radiators is if I request hot water as well, if I only want heating nothing happens, the room thermostat clicks and presumably sends the signal to the boiler etc . The boiler doesn’t flare up.
Any advice would be appreciated.

When you say conventional, now days that could mean a Combi, but assuming not and you have a three way diverter valve this would be the place to check; the room thermostat sends a signal to the diverter which should motor to the CH position; simple method with your symptoms is to turn the boiler off, go listen to the diverter whilst someone else switches the boiler to CH and you should hear a motor noise and an indicator lever should go fully across the slot it rides in (torch light might be handy and a small compact mirror) if that happens correctly, but the boiler does not fire then it still could be an internal microswitch.
Now repeat with hot water (DHW) and CH; this time the diverter will motor to mid position and if the microswitch is faulty, the fact that hot water is called (cylinder stat set above the water temperature) this stat which is a signal that does not use the diverter, then the boiler will fire up and rads get warm.
Sure some Professionals will have better ideas too.

Hi , thanks for getting back to me. Yes it does have a 3way valve.
I will try what you suggested and see what happens.
Thanks again

Has it always done this ?

Malc@67, Had the same in our current place, turned out the boiler Engineer had made a mistake with the wiring from the timer/controller to the 3 port valve.

aeromech3
Tried what you suggested, didn’t respond as it should have done.
Have ordered a replacement. Thanks for the advice.

Neelix,
Hi thanks for for getting back, no it’s only just started doing it.
Other posts are suggesting it might be the diverted valve.

wine~o,
Hi , the wiring is ok as it’s been working perfectly before.
Thanks for the suggestion anyway.

Depending on make (and failure mode as you have not said what actually happens) you can buy just the electrical actuator, but sometimes it is not much more expensive to buy the whole valve and actuator, especially if the old one has been in service for some years, as they get stiff and the rubber degenerates too.
Also between makes some do not fit over another makes copper valve body.