Just to update on what I've done...
After sealing the bonnet cable release grommet and the electrical box beside it, I was still getting a leak developing behind the pedals, specifically on the horizontal ledge that the steering joint goes through, and the odd drop on the accelerator pedal. On the horizontal ledge, it seems to be issuing from the black plate that the pedals are bolted to. I must add that when I was pouring water to test, I was pouring about 1 litre at a time, starting near the centre wiper motor mount, and sloshing it all down towards the exit, making sure it washed around the electrical box and cable grommet.
I then noticed that in the scuttle gutter, near the centre mount for the wiper motor, there is a softer covering panel over the gutter seam, and at this interface, it is quite soft and I can see the water squelching when I press at this point, which is circled yellow in the first picture.
So I decided to do some sealing with silicone at this area, and along other edges of this panel in the scuttle gutter, as seen in the second photo. There is also a cut square in this panel further up the gutter that has what looks like something to mount on it (not present in my car), so I sealed up that as well, also can be seen in the photo.
After testing with more water, I've found that I'm still getting some water in, but it has been curtailed significantly, so have a look at this softer covering panel, particularly where it crosses the seam, and silicone it if you think fit. I think they used some type of bitumen-like adhesive on these panels, which has dried out a bit over the years and shrunk, making a gap.
I didn't do much water testing yesterday, but it was a fine warm day yesterday, but when I was looking around at it today, I noticed a rusty spot behind the electrical box (3rd photo). It is very small, but if there is rust there, there must be a hole, to some unprotected area of metal underneath. It's only a couple of millimetres, but may be enough to let some water in, in some other threads I've seen others find small holes that could cause problems. So I ended up siliconeing this hole, and the whole area of that seam behind the box as well (4th photo). It may be overkill, and there might not be much getting in normally, and I might only be seeing it get in because of the volume of water I've been dumping in and around that area to check for leaks, but hopefully it does the job.
Going to put back what's necessary to be able to drive the car, but will leave some inner trim off for another few weeks, and check after any rainfall to see if the work holds up.
Also, because the scuttle panel was off, I ended up doing the pollen filter but didn't use silicone, I used some butyl tape, after scraping off the foam seal that was on it. I don't think water had ever infiltrated past it, but one side of the seal came off very easily (uphill side), so failure may have been forthcoming. There was also a bit of dirt and leaves underneath the lower filter housing that would
help retain moisture and dirt, so I'm glad I tackled that as well. I also silicone'd up the blank grommet beside the pollen filter.
Finally, just to say I also cleaned surfaces before applying the silicone sealant, with 70% isopropyl alcohol. I really needed to do it because of the problems I had with the first silicone tube I applied which wouldn't set (probably essentially silicone grease applied). But it's always a good idea to clean the surfaces before applying silicone sealant.
