I have a 480k rayburn with 11radators, 8 which we use all the time which when turned up are only warm.tried everything,thermostats on rads,bleeding,sludge remover in system and replaced pump and motorized valve but even after changing valve there doesnt seem to be a lot of hot water going through port A to central heating(can actually hold the pipe)/Hot water is fine.previous to this the system worked fine.Could it be rayburn control.
It could be the system is out of balance.Try turning all the rad valves down to closed position then open each 1 to half a turn. the valve which is the hottest on each rad is the flow valve. starting with the rad furthest from the boiler open flow valve until enough water flows thru to make rad hot. Do this with all the rads, it takes a long time to get right and you may need to go thru the whole process more than once, some may even need the return valve adjusting open more as well.. Good Luck
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.