Jim
Thank you. This did work eventually. On the brief document I posted, the 'space after' on the last two rows was set to 12pt, and when I reset it to 0pt, everything was fine.

Bizarrely, however, on the original, much longer document, the 'space after' was already at 0pt so that didn't work. However, by using the styles drop-down I managed to pick a style which was used on the other cells, and that did work.

So it's solved, but I'm still not sure how!