Just to follow up on this issue, I have recently started experiencing one of the original problems again where initially it feels like the brakes are barely working but on pressing harder they grab too hard, sometimes to the point of locking the driver's side front wheel.
Today I decided to look at the n/s caliper, removed one pad and pushed the pedal to move that piston. But it took a great deal of effort to move it and then to push it back in again. After trying several times it gradually freed up so I put the pad back in and repeated for the other piston, which moved easily right away.
Reassembled and road tested, found the brakes are now fine, so appears to be further sticking piston issues in the OEM calipers I fitted last year.