Right, I've cracked it. For your interest and info, here's what I did.
I found that if I put my cursor after the word 'area.' and pressed Return, the cursor moved just below to start a new paragraph rather than jump to the next page. So I re-typed the following paragraph, which behaved itself by keeping five lines on that page and jumping to the next for the final two lines.
Then, of course, I had one paragraph twice, so I deleted the original one from its moved-down position, then was able to move the graphic up to just underneath the retyped one, to match the Cornish version on the right.
So problem solved -- though I still can't explain it in technical terms. Thanks for your kind suggestions.