do all the rads upstairs heat to the top? have you had the heating on long enough to get to full temp? is there a thermostatic valve in the return on the cylinder?
it might be worth checking to see if the f&e tank has sufficient water in it, if this is ok i would suspect there is either a blockage in the cold feed or the boiler stat is not working correctly.
the chances are that the cold feed is blocked, the most likely place for this is where it enters into the system. You are most likely to see a 15mm pipe cold feed, and a 22mm pipe open vent join the system just before the pump. the blockage will be where the 15mm connects.