We bled one of rads, as suggested, and got the pressure down to below 2 but when we put the heating on it shot up again
When you heat water, it expands.
Manufactures compesate for this by installing an expansion vessel within the boiler casing, normally on the back of the boiler, but that varies with each manufacturer.
Because water cant compress to absorb the pressure, the expansion vessel is filled on one side, with air and seperated from the water by a diaphragm. Water heats, it expands and compresses the air.
Water cools, volume reduces, as does the system pressure.
If your expansion vessel does not have sufficient air, the expanding water increases in pressure rapidly.
Either the tube to the EV is blocked or the air needs recharging.
Isolate the boiler by closing the flow and return valves underneath. Drain the boiler, but not through the PRV. Check the pressure on the EV (car tyre type schrader valve)
Charge to the manufactures guide or label, (allowing for system volume) while keeping the drain valve open to allow water to run out.
You should have pressure in the EV, but nothing on the boiler gauge.
Open the F&R valves, pressurise to 1 bar and turn the heating on.
If you get water from the schrader valve, or you cant pressurise it, your diaphragm is split.