Think I’ve managed to fix it! After sealing the gaps on the underside of the drain hole there was a noticeable difference in the amount of water getting in, noticed I had missed an area on the first attempt, after the second attempt have been totally dry for two weeks, did an extreme weather test this weekend with buckets and hoses and no water made it in, no nappy needed.
From reading on another thread it seems to be more of a
problem on 2018 reg cars, my theory is that at the factory they forgot to seal / or there was a defect with the metal panel on this side (the hole on other side with no water ingress looks a lot better), water makes it through the gap, when it freezes it expands causing the gap to get bigger enabling more water can get through freeze, expand and repeat in some cases causing the metal to split /corrode.
https://www.seatcupra.net/forums/threads/water-ingress-into-drivers-footwell.471576/post-5025681
Leaky side
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Non leaky side
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Definitely worth checking the condition of the hole, simplest way I've found to do this is to access the area from below, above the wheel arch by tuning the wheel full lock gives better access. Putting a smartphone recording in video mode with the light on through the gap in the wheel liner, waggle it around and freeze frame the hole afterwards.
I sealed mine with tiger seal (cheaper version Puraflex 40 PU Sealant). In hindsight it would have been easier to do by taking the wheel off and potentially the wheel liners, managed to get to mine from above by lifting up the windscreen scuttle tray removing a clip and plastic pull rivet and lifting it up (be careful it breaks easily as I found out although only cosmetic) and from below by reaching my hand around the wheel liner and working by feel, hence why it looks like a dogs dinner.
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Top tips: uses disposable gloves and throw away after each application, have white spirit at the ready to wipe off any spillages on the body work while it’s still wet, video the area after dry to check for missed bits, don’t get any sealant on your phone.
Pretty sure it if fixed but will post back if it returns, hope this helps.