Ok, great, thanks to all for input, understand about tank needs to be higher, but why is it filling when bleeding rads (only top ones?) what`s sealed system mean.
because when you are opening the bleed screw instead off letting air out (air expelled due to higher water coming in to rad and pushing air out)you are in fact letting air in releasing the vacuum that is preventing the water which now wants to spill into the tank due to the fact that the "highest" water is in the loft rads.
the system you have called "open vented" its open to the atmosphere a "sealed" system is sealed to the atmosphere and pressurized and has added safety components
"hope I've explained that ok