So, having left the now-sealed wheelarch grommet overnight, we have large pools of standing water in the lane, so an ideal chance to test the effectiveness of the repair, thus:
I'm kneeling in the space normally occupied by the passenger seat, looking intently at the floor, sill and footwell, whilst being driven along through as many puddles as possible!
Some small amount of water, but I'm not sure whether this is fresh or if the bumpy ride is just dislodging earlier ingress. However (and this is the startling bit...):
Where the black, bitumen-like sound deadening is cracked as it goes over the gulleys in the floor, there are signs of water. Initially I put this down to the vibration, but not quite sure.
Pulled the whole layer up, front and back, to find each of the gulleys has a crescent-shaped cut at the rearward end, through which it is possible to see the road below. Each of these is - to a greater or lesser extent - part blocked with fine, muddy silt, very similar to that found elsewhere on the floor.
Not only that, but there are four round holes, 10mm dia. approximately, in the rear floorpan.
I cleaned the silt away and dried thoroughly, then had another puddle-splashing drive. Surprise, surprise, water comes in through these cuts...
- Are they there for a reason?
- Surely they would (should?) be covered by underseal?
- When they block up (as mine had) they can't function as drains if water is getting in elsewhere.
The whole area has been thoroughly cleaned and dried (again!) and all the holes taped over to stop any more water. Tomorrow will see another test, having eliminated these from the sieve