Take the rads into the garden and get jiggy with hosepipe. INvert rads when half full.
Connect hose pipe to rad valve union - water should p out of the other valve. Then close all other rads and it has to do it via the boiler. NB header tank might overflow, but it shouldn't if you have an open end.
If that doesn't work change the pump - put a 6metre head one in. Cheap option compared to taking boards up. Check pump valves aren't blocked . You can turn it down..
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