My Isar is not igniting, and having followed the fault finding
flowchart there are no error codes either - in fact nothing on the
digital display. There is power and gas to the boiler. It's not working
on DHW or heating

According to the chart, if no error codes are shown or no '0' on the
display (to show that it is on standby) then there is not a problem
with the boiler - but I should contact the installer!

Is there anything that I should suspect to be more like to be causing
this problem?

I don't know if this is significant, but the CH has been off all summer
and two weeks ago I noticed the pressure guage was near 0 so I let
some more water into the CH system. Obviously a bit of a leak
somewhere, but as the heating is not on I thought it would be a job for
another day. Having read the user guide it does mention that the boiler
should not be iignited if the presuure has gone to 0 and been refilled.

But this was two weeks ago and the boiler was working OK since, and the
CH is off anyway. So I don't know how this would affect things.

Cheers

dg