I had Musky's problem years ago on a Yamaha XS 650. Front brake working fine, then binds on. Realised what was happening, so saved the disc. Roadside fix is to slack off the feed pipe, sacrifice a few dribbles of fluid and push the caliper back. Re tighten the feed, ride carefully and avoid using that bad brake unless you have to.
Simple cause, in this case alloy crap blocking return hole in master cylinder. Bet Musky's is a similar manufacturing fault, or a coincidental failure of a rubber hose playing the one way valve trick.
Swarfy.
PS. Nice one Bergs, Fine Dining in Buxton. Ferodo Factory not far away.....just out of interest.