Hi Steve,
Car is there at the moment. Will let you know final outcome but here is the feedback they gave me yesterday.
Many (!) 991's are having similar issues (they 'only' did 6 in recent weeks), mine is a (2016 S coupe, but there are also issues with cabs); partly due to aluminium chassis versus previous steel chassis. These have to be bonded instead of welded and apparently the question is whether Porsche are using enough glue..
Majority of leaks start on lh side of the car and are usually caused by a faulty door seal. Couple of hundred pounds for the seal alone and a sh... job to replace according to them.
Process; take out seat and carpets (to dry), check control box(more info down), then hose car down until water starts to appear, usually door seal.
Fix it. Dehumidify the car for a couple days and re install.
I am hoping that by drying the majority out myself the electronics are not damaged. So far they have seen 1 car where it needed replacement, but 'that car was flooded'. If so, the only way to go is Porsche as they need to recode the new electronics box to the car (theft safety I guess and they require both keys). Still the independent would do the 'donkey work' given high hourly labour rates at Porsche themselves.
There are 2 control boxes; the one affected in my case (passenger rear footwell) controls the back side of the car; whereas apparently there is another one in the front footwell..That footwell was dry as both are separated with a cross member that held out the water from moving to the front footwell.
Equally, I thought the water ingress was due to the recent bad weather but the actual issue is that it's a process of months as water slowly builds up until the tell tale signs; wet/ soaked carpets, dodgy electronics. It can easily be multiple litres of water and once electronics are submerged it's going to be very expensive. I am crossing my fingers at the moment. Yesterday all electronics were working but who knows. Funnily (..) enough, I had them quickly check over the car about 3 months ago as last summer it was misting up from time to time. With a quick check we could not see anything (and certainly not if it builds up from the bottom instead of pouring down visibly on the inside..) so I thought nothing of it.
Finally, one would expect Porsche to do a recall or at least acknowledge the issue; probably too expensive or they feel the number of issues versus the total of cars produced doesn't justify. Anyway..