the long bolt fell unscrewed out of the crank and caught on the drive side stay.
luckily i was pedalling slowly.
it's gouged a bit out of the weave and the area is probably less than the size of a 5p coin.
it hasn't created a hole.
no idea why it happened as i've been riding it for a couple of years no problems, but first ride of the year (yes...) and i'm gutted.
is a repair possible?