I posted this question about 18 months ago and had many helpful replies,thank you all.
Shortly after installation I noticed the boiler pressure was rising slowly(it got to 3 Bar).After bleeding the rads.(it's only the two u/stairs ones which need bleeding)the pressure dropped,as expected, but then started creeping up again,bleeding the rads led to a pressure drop,then a pressure increase etc.
I contacted the installer who scratched his head and avised me to call Ideal
I telephoned Ideal,they sent an engineer.
Over a week,at first disconnecting the old filling loop which had been left in, then changing one part at a time,he changed the heat exchanger,the expansion vessel? - and the pressure gauge.
This hasn't cured the problem.Over time,during which the boiler has been serviced, I've monitored the pressure and as the pressure rises I've bled the rads which reduces the pressure,I've done this about once a month. This last time the pressure gauge dropped to zero and the boiler shut down - advising me to re-pressure the system or call an engineer.As an experiment I did neither, but waited,after 5 hrs.the boiler had self pressured up to 1Bar !
Before I contact Ideal Standard again can anyone give me some idea what might be happening?
ps.the filling loop on the boiler is not left connected so that can't be the problem