Flipping cheapo Aldi bike covers! It's just glued (badly) down the seam, which the mirror is pressed up against. I guess it was only a matter of time.
Wouldn't bother me but I've kept the bike clean and dry while I've been unable to ride and when I got home it was soaked all down that side. Grrr!
Wonder if it's a SuperGlue job or if it's time to spend more that £5 on a bike cover