Why does pressure keep rising to 3 bar on Worcester Heatslave 20/25?

I have a 20 year old Worcester Heatslave 20/25. The heating and hot water are switched off. The pressure gauge (fitted a couple of months ago) is showing 3 bar and dumping water through the overflow outside. The filling loop has been replaced and I can confirm there is no water coming through it. There is a sound of flowing water into the bolier. If the filling loop is isolated what is causing the pressure gauge to read 3 bar? Even if I purge it down to 1 bar, it soon climbs back up.