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Hmmm, seemed like the photos uploaded to that particular gallery were particularly large (some of them were 16Mb!).
Gallery settings were the same, and the photos behaved in the same way if added to another gallery.
I deleted the originals and re-uploaded them using the Imsanity plugin to compress and reduce the image size, and now the gallery works OK.
Consider this resolved, thanks.
Sorry, your reply prompted me to realise that there are different templates that can be used for the gallery. Choosing the “Template 2” option has solved the issue.
Apologies for the oversight – please feel free to close this as “resolved”.