To those who may read this merry Christmas in advance. Update as promised: it was as expected. Water ingress to the shark fin area causing the issue. The dealers garage had my car in for brake change so they investigated themselves and found the water ingress issue. I’d pointed them in that direction that is to the great information found on this and other forums so thanks to everyone who’s experienced this issue and posted about the results in the past. I’m on the way to pick up my car. They’ve ordered the part and I’ll take it back in for a day to install when I get a chance.