Boiler not heating water sufficiently

Potterton Prima F boiler, (approx 6 years old) few months ago, heard lots of noises from the boiler, realised that it was due to air in the system and bled all radiators - there was a lot of air in the downstairs radiator, boiler worked fine for a few weeks and then started to emit some sort of wheezing noises, then as of yesterday only heats up water to lukewarm.
(Have been forced to bleed upstairs radiators every couple of days for the last few weeks as air still seems to be getting in from somewhere.)
Now both the central heating radiators and the hot water supply only get lukewarm.
Any help appreciated. Thanks

I'll guess,......you have an open vented system, and the feed from the header tank is blocked. Tie the ballcock up and drain some water out of the system, see if the water level goes down.

Hi Oilman, thanks for the tip. Did check and drain water sufficiently to ensure there is no problem with the feed. All of the upstairs radiators and 2 of the downstairs ones now work properly. However the big ones in the lounge and in the passageway downstairs only get lukewarm. Did find a pinhole leak in the passageway radiator which I have now stopped but that explains the air in the system previously. Have opened and shut the valves on both radiators a few times and even tapped them to ensure that they are not stuck. Bleeding them seems to let out the water correctly so it appears there is sufficient water in each radiator.

There is a Myson DC53 pump making a bit of a whirring sound and if I change from setting 2 to 3 or to 1, the sound changes from within the pump. Tapped the pump sharply, a few times, with a mallet as suggesed in the forums and even opened the centre bolt and shut it again in case the pump was stuck. Tried to reduce the water flow in the upstairs radiators by turning down the radiators in the hope of forcing the hot water downstairs but still no luck with the two big radiators downstairs.

Also there is only lukewarm hot water even though both the CH and HW are turned on together. Any further help would be appreciated. Thanks.