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Señor_Bear PhilDAS and Scillu.Suffolk were right
I cleaned out the dropouts with brake cleaner.I also scored the paint from surface of the dérailleur hanger into which the quick release could not embed (unlike the other three aluminium surfaces of the dropouts themselves), and tightened the hanger bolt.
But basically it was a case of dirty dropouts.
Thanks very much indeed. I junked a carbon frame for this! Two of my local bike shops (rather non-sportive admittedly) did not know what was going on. This needs to be shouted from rooftops. I'll post to the other thread mentioned above.
user80021
Don't just tighten it up, take it out, clean it and also clean the faces of the frame where it attaches (the inside and outside of the dropouts).