Boiler pressure too high?

I have a Worcester Bosch 280 combi boiler. The pressure on the scale is way past 4 bar. I have reduced the pressure once back to 1.5 and made sure the filling loop is shut off, but it has crept back up. There is a screw valve where the loop joins the pipe into the boiler, but i cannot for the life of me turn it off! Have looked at other posts, but still not sure what to do.