My request for advice about a replacement handbrake spring can safely be ignored. As I hoped, working the lever by hand with copious penetrating fluid got the lever working very freely again. After that I continued to change the discs and pads, except that I found that both pistons had suffered abuse, probably while a chain of 'quick' repair shops here had butchered the pistons in their attempt to retract them. Obvious marks (deep) from the use of a drift of some sort to rotate them, sadly indicating that they had been attacked in both directions. At that point I decided to leave everything in place until new calipers arrive.