If your hot water is ok and no fault codes are showing when the radiators are on then the boiler will probably be working ok. It could be that the radiators are undersized for the rooms and not able to bring the room temp up to a reasonable temperature.
I would suggest you have your boiler serviced by a independent gas safe registered engineer who can adjust your boiler and system as needed or give you an opinion on why your system is performing below par.
It may need to be chemically cleaned, or could be a gas supply problem.
A service I would guess to £60 to £65 in your part of the country and a descale/desludging £300 to £350.with a descaling pump.
dont be tempted to use friends or relatives unless they are gas safe registered and know what they are doing.