Another day in the shed... a day of revelations. What I found was that all three of my caliper mounting brackets have different numbers on them. One has 5 5 stamped on it, one has 8 8 stamped on it and one has M2 stamped on one side and an 11 on the other. Of the three, one is distinctly asymmetrical in the vertical plane, ie it bends more one way than the other. The other two are symmetrical. Also, one is of thicker diameter than the other two at the end where the pivot pin goes through it. Also realised that the mudguard bracket is not the same as the one originally on the bike. So ... combination of narrower hub on 550 forks, different caliper pivot arms and non standard muguard bracket seem to have thrown things well out. Spent all day messing about with different combinations of pivot arms, pivot pin brackets , placement of mudguard bracket ... and got there in the end . Now have a left and a right hand caliper each with a gap of 0.06 inch between the fixed pad and the disc and a freely rotating wheel.
On both sides the mudguard bracket is fitted as standard .. ie fork, mudguard bracket, pivot pin bracket on the right and fork, pivot pin bracket, mudguard bracket on the left.
On the right hand side I didnt need any shims or to file anything down to get it right in the end, just had to add some washers to the ajusting screw to give the spring some compression.
On the left hand side all I had to do was file down the leading edge of the pivot pin bracket (see photo 1).
The second and third photos show the calipers correcty positioned and adjusted. So, thank goodness , got there in the end .
Thanks to BryanJ, Odjob and CrazyP for holding my hand through it all
