Potterton suprima constantly firing up

[charmOl]

Boiler works ok when heating is on at the programmer but as soon as I call for heat at the room thermostat the boiler then continually comes on for a couple of minutes then shuts down and repeats doing this until I turn the room thermostat off.
Thanks CBW & jackthom for your replies

 

[CBW]

Could be a faulty room stat. What room stat is it? Do you have any test equipment?

 

[charmOl]

It’s a potterton one but not sure of the model. I’ve added a photo of it.
I’ve got a self ranging multimeter.