Hi . on your radiators is the bleed valve at one end. Open one of these and have a towel and small pot . open the bleed valve ( with a radiator key) and release water till the pressure drops to about 1.2 bar.
Be sure not to open the valve fully but just a half turn till the water starts coming out
OK thank you will find the radiator key now.
Is the high pressure the reason for the overflow outside to be dripping a lot too?
Yes. There is areas on for this. Inside the boiler is a pressure vessel . like a small metal tank with a ball on inside. This is to take up the expanded heated water. If the vessel goes flat ( should be at. 1,5 bar ) or it has torn . the water instead of expanding into this vessel has nothing to do except pressurise too high . the pressure gauge then goes up to 3 bar ( settingvforvthe safety valve) and releases this pressure, with the water out of the safety to out side. It then becomes a circle , of refilling , over pressurising , dumping and so on. The thing to do is check the pressure of the expansion vessel ( I use a car tyre gauge) to check its right . but also once the safety valves have opened, sometimes they never seal back up properly ( due to gunk ) and will need replacing .
Thank you so much I will do the radiator bleeding and then get a local boiler firm in to check if the safety valves need replacing.