Manky bolt.
I have some rebuild kits on the way as I was going to strip and clean the calipers. Bernie (the bolt) is obviously going to have to come out so I can see what lies beneath. Like a Mumbai street meal, you won't know until later.
Has anyone used the repair kits that include a drill, tap and new nipple and carrier? I would normally replace the caliper, but they are very expensive and/or not available.