Potterton Puma 80 overflow keeps running?

Hello. My approx. 8 year old combi occasionally loses a bit of pressure and is normally sorted by using the loop to build pressure up. I did this yesterday, but the needle rose to 3.5 bar immediately and the overflow poured out water.I tried taking about 3 or 4 pints from a radiator to try to reduce pressure but no joy ! It remains around 3.5 and the overflow is still running. Heating and hot water both operating normally at present. Any advice welcomed to recuce pressure and stop the overflow. Thanks a lot.