Boiler / central heating problem Potterton Profile

[alfa.romeo164]

I have a problem with my central heating, which is driving me crazy!. The boiler (a Potterton Profile) just does not stay on long enough to heat the bedroom rads. Hot water seems to be fine. But when I try to run the heating the bolier clicks, whirs for about 15 secs then repeats this process several times before eventually firing up. However, when it evenutally does start, it only runs for a few minutes - enough to get the bathroom rad and a landing rad hot - before shutting down again. The fan then runs for quite some time before stopping. The rest button is often triggered too.
When the boiler cuts out and the pump stops running, I can hear a rush of water and gurglings which seem to be going through the pipe with the pump on it. The problem seemed to start after I fitted an additional rad, but I have flushed the system as much as I can (I took the main bedrrom rad off completely and blasted it through with a garden hose) and all the rads are properly bled.
The system is a fully puimped, but not sealed one, with the pump pumping downwards. It is all on the first floor and the header tank is only about 2ft above the boiler. There is also a strnge looking fitting at the top of the fill - return loop, which I take to be some sort of air seperator (though as it appears to be sealed and has no valve on it I am not sure how it works)
Any clues on how to fix this would be gratefully received. Thanks, Mike

 

[corgiman]

how did you "flush" the system?

 

[alfa.romeo164]

I disconnected a joint into one of the rads and connected that via pipe to a cold water tap, at the same time as opening the drain valve....

 

[alfa.romeo164]

Now the boiler keeps locking out and the return pipe stays cold most of the time then suddenly gets hot, though the rads are not. Also when I put my hand in the supply tank, hot water is flowing OUT of the supply feed outlet! - but the level in the tank is not changing.....