My combi boiler still sounds as though it is working although because of the thermostat settings there is no heat being produced through the radiators. It is the same when switched off via the time clock
It may have "comfort" mode, which keeps main heat exchanger hot for instant hot water. In Worcester you switch that off by pressing "ECO" button for few seconds until it lights up green.
My advice is change your combo boiler to any of 10 year warranty ones like Ideal, Baxi and etc. to repair boiler parts just cost too much and if boiler is old it’s probably won’t last long anyway. When they start getting issues it’s time to change… you save more money on long run