I pulled off the os calliper and found the cover neatly, symmetrically, torn

I initially thought the disc had somehow cut through it but that didn’t line up. Looking at the pads they have a couple of raised spots on them, that had caused the problem.
Both pads were the same. Maybe I’d bought the wrong pads ? Nope. These two are both outboard pads. The inboard ones have the spots further apart (top):
In my haste at the weekend I’d not noticed this and fitted both outboard pads on one side and both inboard on the other. BMW pads normally have a large 3-pronged clip on the inboard pad that makes it blindingly obvious. These MINI pads are identical apart from the spacing of these spots.
Reassembled and all is well
