Pressure to be at 1 to 1.5 when cold. if still goes gigh than your filling loop letting water in to the system thus pressure is high
If pressure is i to 1.5 and goes when heating is on than your expension vessel if faulty, may try to fill pressure 0.9bar when system to zero. if still cannot work than vessel is faulty, need either replace it or add another on in any heating pipes anywhere in the house will work.
k, December 2010