This seems like a "S" plan from your description but obviously, it is difficult to be sure.
The most likely culprit is with a faulty zone valve. You say the boiler is on constantly, but are either the towel rail or the other rads constantly heated too? If so it could be a stuck valve. Power down the heating system so it is totally off, and operate the manual lever on the end of each valve and feel if the gears are moving smoothly. If the lever is floppy, the valve is stuck open.
Turn the power on, and with the heating and water set to off, and the fault is present, examine the valves again. If either is floppy, the fault lies elsewhere, if the resistance can be felt, the probability is one of the micro switches in the valve is causing trouble.
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