Many, many thanks to PeterBreis0807, Peggy and VikingOSX. And also to VikingOSX for the link to the update for iWork '09 to 4.3 (Pages), 2.3 (Numbers) and 5.3 (Keynote).
While at home for a couple of days last week (yes, I travel quite a lot so it sometimes takes me some time to respond) I managed to find my original iWork '09 disk purchased when it was boxed with as far as I remember Snow Leopard, loaded it under Yosemite, which then told me it wouldn't run, updated it through the link supplied by VikingOSX and lo and behold, they all work again at last.
I fully agree with your comments on the pain of using the two versions on the same machine or worse a population of machines at different levels of operating system and software generally - I just have to be very vigilant. Nonetheless I prefer to keep the new versions for occasional tests, and to find out if things I managed to do easily on Pages 4.3 are even possible on Pages 5.5 and if so, how.
So, many thanks once again!