The only two jobs post Donington were to change the sticking n/s/f caliper and to adjust the n/s roll cage side bar section that wasn’t tight up against its mount location. Both straight forward to complete.
A new old stock caliper was sourced via eBay and fitted. Both front calipers bled and the sticking problem is solved.
Just loosening the side bar section bolts on the cage then re-seating the bars properly in their location cups before re-tightening the bolts sorts the cage issue.