System pressure is 1.0 bar when ambient, rising to about 1.4 bar when up to the flow setpoint of 72. The circulating pump is fitted after the boiler (1.5 meter in terms of length of pipework) in the flow primary and before the three two port valves for the three zones I have. The radiator in question is the first radiator on the downstairs leg of the heating zone so absolutely not the highest point in the system. The highest point in the system would be the flow pipe at the hot water cylinder which has an automatic air vent fitted (as does the flow pipe just above the boiler flow output.