Well. I resolved this by duplicating the page, removing the accordion element and hey presto - all fine. Now I know that it was not the accordion by itself, as I use this on other pages that google has not issued mobile warnings on - so this is my basic understanding. Some pages were constructed during the Fluid engine transition and were never functioning well after. Only by reproducing the page now things are more settled, am I seeing these errors disappear.
I have now run the same test on other pages with - specifically the Content too wide for screen warnings - and am seeing google remove the warnings within 24 hours. So after 4 months of seeing pages slip away, they are slowly returning.
Now, off to finish the other mobility warnings....