If you have a conventional boiler and HW cylinder .. AND a Mid-position valve (in cupboard pipework .. with a silver boxy top cover) to direct HW from boiler output to either the radiators or HW cylinder .. then it's most likely a leaky or partially stuck/worn Mid-position valve assy. That will need replacing.
If not, you had better describe the system.
OK ... A little more info there .....
Is that radiator the nearest to the boiler? If so .. it's thermal leakage from the diverter or control in the boiler.
How old is the boiler? And the radiator is heating up when hot water drawn .. right?
OK ... If you want to fix it before it breaks (it 'could' go on like this for a long time tho) .. better get a plumber who know those boilers to clean/check the components and possibly replace the valve and replace any seals same time.
9 years is not bad with no major component replacements tho.
Thanks a million AlBags, nice one. Oh dear, more expense but all for a good cause. You've cracked the mystery and now it can be put right.
Thank you soooo much x pusskins