Nothing seemed to be in the way or rubbing against it (that I could see).
Given that derailleurs are black science I basically ignored it until I could get someone to have a look at it whilst I was in motion. It turns out that the chain has stretched to the point where the derailleur is folding back on itself in some combinations, particularly when I'm in the top of the middle ring.
Oh well , I can put up with it for 500Ks or so until I replace the chain and cassette. :)