my halstead ace high boiler does'nt let regulate the heating water ?

a couple of wks ago when i turned the heating on the pressure would start on 1 bar as normal and to 2.5 bar when rads were hot as normal and when they cooled the pressure would drop to about 0.5 bar so id have to top it up.. then it changed and when the rads had cooled the pressure would stay at about 2.5 bar or a little more and it drips constantly drips from the overflow now and from the internal overflow inside the boiler which makes my kitchen side wet in the morning..