A radiator with air trapped inside cannot fill with hot water, so part of it stays cold and the room never quite warms up. Bleeding that air out is a five-minute job that costs nothing, needs no plumber, and lets your heating do its work without the boiler running longer to compensate.
Spotting the ones that need it
With the heating on, run your hand up each radiator. If it is hot at the bottom but cold across the top, air is trapped in the upper section and it needs bleeding. Gurgling noises are another giveaway. Cold at the bottom with a warm top is a different problem, usually sludge rather than air, which is covered below.
Doing it safely
Turn the heating off and let the radiators cool so you are not dealing with scalding water. Find the small square bleed valve at the top corner, hold a cloth and a cup beneath it, and turn the radiator key anticlockwise a quarter turn or so. You will hear air hiss out; when water starts to dribble steadily instead of spitting, the air is gone, so close the valve again snugly. Work round the house, top floor radiators last, as air collects highest.
Check the pressure afterwards
Letting air and water out can drop the pressure in a sealed combi system. Glance at the boiler's pressure gauge once you are done, and if it has fallen below the normal band, usually marked on the dial, top it up using the filling loop as the boiler manual describes. It is a simple step but skipping it can leave the boiler locked out.
When bleeding is not the answer
If a radiator is cold at the bottom while the top is warm, the trouble is sludge, a build-up of rust and debris settling inside, and bleeding will not help. That calls for a power-flush or chemical clean of the system, a job for a heating engineer. Keeping the system free of sludge, and keeping radiators bled, both help it run at the lower flow temperature that makes a condensing boiler efficient.