As you have an open vented system, it probably has some sludge and sediment in it, unless it was recently powerflushed, which may be obstructing correct flow.
It may help to give it a simple chemical clean, which will only cost you about £15.
Sentinel X400 can be added and pumped round in normal operation for about 4 weeks before being drained out. On the first day, it may help to turn off all your radiators except one, and go through the house doing that to maximise flow through each radiator in turn.
I bet you will get an alarming amount of dirt out.
It may help to give it a simple chemical clean, which will only cost you about £15.
Sentinel X400 can be added and pumped round in normal operation for about 4 weeks before being drained out. On the first day, it may help to turn off all your radiators except one, and go through the house doing that to maximise flow through each radiator in turn.
I bet you will get an alarming amount of dirt out.