So, in order to change/check the shim sizes you need to remove the rocker shafts after having removed the covers. I was slightly concerned that because the cover holds a bearing that the shaft would become unstable but it doesn't appear to make any difference. With the covers off you can screw a M5 screw into the shaft and then pull out the shaft, being careful to grab the shims and thrust washers as they fall and noting carefully what positions they are in.
With the inlet rocker on the bench, measured the shim thickness and calculated the corresponding amount to sand off. Apparently this is customary practise for ducati shims and it worked pretty well and after two attempts I got he clearance on the opening shims to within spec.
I am considering the closing shims as good and the horizontal cylinder was also just within spec so that was left as well. On the opening exhaust rocker the end of rocker shims were all at one end without any thrust washer! I think this has been badly put together so I have moved a thrust washer and shim to the other end, I need to see what the spec is on the end float for these shafts.