Thanks for replies.
More info - it's just a studio flat on middle floor of converted 3-storey house. 4 radiators in total - 3 small ones (in toilet room, bathroom, kitchen) and 1 larger one in living/bedroom. There's a room thermostat on wall in tiny area between flat entrance door and the 3 doors leading to toilet room, bathroom, kitchen.
In case it helps, a couple of things I've noticed - to get larger living room radiator to heat up properly it seems room thermostat needs to be set to at least 22/23 degrees (which means radiator is either too hot or not hot enough). Used bleed key on it but no difference. Also, with boiler set to "Hot water only", individual radiators seem to randomly go from mildly warm to cold, eg the other day I put a t-shirt on warmish bathroom radiator to dry, but the following morning the radiator was stone cold and the t-shirt was still damp.