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Thank you, Bibblebub and Buenchico. There's good news and bad news; first, as you had guessed, I had forgotten that I needed to highlight first....however, the page concerned is a photo with some text underneath it. When I try to highlight the picture it just turns into a sort of negative version of itself. I have managed to copy it correctly, but it hasn't taken the whole page including text, just the picture. Copying just the text is fine.
I did this last year with similar pages with no problem,and I can't think what is different now.