Possible header tank feed blocked?

Hi
I've got a CH problem that I'd appreciate a comment on.
The boiler fires up for a few seconds and then shuts down. I eventually got it working for the day by bleeding (several times) the radiators and pump.
Today the problem re-occurred. The downstairs radiators are full of water but the upstairs ones will bleed out some air and then stop. The bleed screw can be taken out and no water comes out, upstairs.
The pump is OK
The header tank has water in.
I have rotated all the isolating valves in case scale was sticking to them.
I am suspecting there is a blockage between the header tank and system. Is this sounding reasonable?
Whilst fiddling around I noticed the 3 way valve on the HW cylinder does not appear to work and the manual override does not seem connected to anything I\\\'m not sure if this is connected to my main problem?
Is there something I\\\'m missing or is it time to get in a professional? Trying to find a plumber in the UK at a weekend is not easy!!

Does sound like the sludge blockage in cold feed scenario You can find me in Sussex anytime

Just noticed what my wife added on about plumbers. Sussex is a bit far away from Bedfordshire. I am waiting a reply back from our regular plumber or a recommended one with whom I left messages (Thursday/Friday). This is not an emergency call out situation so it might be tomorrow before I can talk to someone. But I hate doing nothing also my ear gets a beating from "She who must be Obeyed"
Many Thanks
Martin