Thank-you both for the prompt replies.
I'm currently using a very old Wacom Bamboo, with legacy drivers. It is actually very stable in Mojave, which is quite surprising. I just have this issue in Pages. It's never been much of an issue before, because I've only really used Pages for writing short notes, pasting from other sources etc. However, I now have a bigger project to start and can't use the trackpad for something so big.
While waiting for a response, I did try selecting text in Affinity Photo, within a text frame. It behaved as expected, so that's encouraging.
I think I should probably just go for it.
Thanks gain, to both of you
e