I
iateyoubutler
I had this problem with my system, all 5 upstairs radiators were red hot when the hot water was on, and if I turned them all off the problem just moved downstairs. This was all traced eventually to the bathroom radiator return being in between the cylinder return, and the boiler - done that way just out of laziness when it was first fitted (I would think). After re-routing the cylinder return by about 6ft and teeing in again about a foot away from the boiler it made a massive difference, the hot water could be on all day and every radiator would remain stone cold.
That was all it took......
That was all it took......