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I hadn't realised this as the pages were loading fine in the browser and everything seemed OK, but a strange coding error was causing IIS to return this error.
Because of this, most of the pages from my site have gone 'supplemental' in the Google SERPS. I have fixed the error now and all pages are correctly returning 'OK'.
I can see Google happily crawling my site now - how long can I expect it to take for things to return to normal?