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This seems a very good suggestion, thanks. Anything that I can rely on to be properly (if not 100% truly) flat, that will span the swinging arms and extend as far fore & aft as possible, will tell me whether it is twisted and if so, by about how much. I'll do this. I'm hoping it turns out to be more or less untwisted, and that the whole issue is due to isolastic rubbers being buggered. I was expecting to be replacing them anyway.