I'd fitted some new Ferodo brake pads, I had to file a considerable amount of protective coating from the sides of the pads to a allow the pad to travel freely in the caliper, this is pretty common with new Chinesium pads but I was a little shocked with the Ferodo allegedly European brand.
Alas as the pads started to wear down it reached the original sticking point again and the brake started to bind ever so slightly, not so as you could hear it, I could only detect the wheel temperature difference after a good 20 miles though. The vibration had returned.
The piston in the caliper was at its original sticking point.
I gave in and fitted a new brake caliper, problem gone.