wrote:
Boiler is in daughter's house. System has 7 single panel radiators.

With the cold pressure on 1.2 bar, it rises pretty quickly to about 2.5 bar when hot.

I found my neighbour has exactly the same boiler and that barely moves -
a couple of tenths of a bar.

Is the pressure rise anything to be concerned about, and is there a
logical reason why they would they be so different?

Most likely a failing (or failed) expansion vessel. If it is built into
the boiler it's usually a lot cheaper and easier to add a new external
expansion vessel than to replace the original.

The installation manual for your boiler should contain details on how to
check the pressure of the expansion vessel. If water comes out of the
schrader valve it's a sign that the diaphragm has failed.

Tim