After standing for a while as no longer needed for commuting, I was about to put the battery back on the car, when I found the passenger carpet was wet.

That led to discovering water in the fusebox!
I cleaned all the places, gutters, etc under the bonnet yesteray hoping that I was solved. However, after a night of heavy rain I've found that the A pillar is actually wet, almost from the top.
Hhhmm, dare I risk trying to get the A pillar cover off, bearing in mind there's an airbag in there?
Not sure what I would find / be able to do re fixing a leak? Is there a sealant inside maybe?
The car doesn't have a sunroof - Googling shows a lot of people get leaky sunroof drains.
The windscreen appears to be original as it has 'BMW' on it at the bottom.