Radiators work even when heating is turned off

thor

I had something similar where turning down the main thermostat did not turn off the heating consistently. Then one day the rotary dial of the thermostat went completely loose and I was forced to open it up to see what had gone wrong. I found that the plastic bit which attaches the dial to the rotary control had broken off and in all likelihood was probably the reason whay it was malfunctioning in the first place.
I have now ordered a replacement and in the meantime am just operating the control without the dial by turning it on and off directly with my fingers.
I think this could be another possible cause. The op may have thought they had turned the heating off but in actuality the thermostat was still on.

plumb1_2

The fitter could have locked the zone valve open when draining the system, and forgot to move the lever back after filling. Check the zone valve.