"Before putting the car away in March I washed it but obviously hadn't dried the wheels and discs".
It';s better not to wash the car at all than put it away wet. I have no problems with my Plus 4 but I shampoo it, rinse off with the hosepipe with a sprinkler rose, dry with a leaf blower (never use a leather), then take it for a damn good thrashing, including heavy braking and applying the handbrake on the move. After that put it back in the garage (without an indoor cover which gets damp) and park with the handbrake off and the CTEK connected. Job done.
