In my original post I mentioned what I described as a misalignment of focuser and secondary - but hadn't been able to work out how to attach the relevant diagram. (Not quite sure why, but there was an 'upload' feature evident tonight which I don't recall being available the other night.) If you can see the diagram it means I've worked out how to attach such things.
I'm looking for ideas as to why this apparent misalignment should be and how it can be overcome.