My upstairs radiators are only warm at the bottom, but when I bleed them they only let out a small amount of air, never fill up and all the radiators upstairs start bubbling. We have a combi boiler (oldy but goody) Worcester Bosch Highflow 400 feeding 5 downstair radiators (which are all piping hot and fill when bleeding - downstairs hall seems to be full of air lots more since the upstairs has 'emptied') and allegedly feeding 4 upstairs. Am I missing a trick? Do I need to put more water into the radiator system? Or should I call out a plumber to resolve the issue? Any advice, anybody?

Try balancing your system by closing the valve a few turns (opposite side to thermostatic valve) on the hotter radiators. This will give a better flow of water to the offending radiator.

Check the pressure in the heating system. Usually it is a round gauge that measures both temperature and pressure of the water in the system. The pressure will most likely be on a scale labeled psi and altitude. The altitude reading is in feet which indicates how far the water will rise in an open pipe. This should be set about 3 feet higher than the vertical distance from the gauge to the top of the highest radiator. (Assuming the gauge is working properly :D) If there isn't enough pressure in the system the water will not fill the upper radiators no matter how much you bleed them. Possibly your PRV (pressure reducing valve) is bad or misadjusted. Or your makeup water could be shut off. A new PRV will come set somewhere around 12 psi or about 25 feet altitude, so if you have a lot of height in your system it could require adjusting.