my ideal isar he35 has been fine for the last 6 weeks since fitting
until this week, i noticed the pressure had dropped to 0.75 bar from
the usual 1 bar (recomended when cold)

Probably a dodgy or undersized expansion vessel, or simply not inflated to
the correct pressure before filling the system. With a pressure rise of at
least 1.25 bar, you are quite likely to run into problems.

Given the new installation, it could be either a manufacturing fault,
installation fault (not checking charge pressure), or a design fault by the
installing engineer who didn't calculate the capacity of the system and
relied on the inbuilt vessel to accommodate expansion, when it was not up to
the task and should have had an additional external vessel fitted.

Do you have a large house by any chance?

Christian.