I thought I'd report back that I did indeed 'bite the bullet' and update from 10.6.8 to 10.10.5. The update went relatively smoothly and required the inevitable update of secondary drivers and software. The new OS and its associated Mac apps is quite different in many ways, so a lot of searching and googling was necessary to find out how to do some things which one takes for granted with 10.6.8, like how to get out of full-screen mode when there is no green button, and how to find internet radio in iTunes, and stuff like that. But all-in-all after a day of faffing around, only an old (PowerPC) copy of Excel had to be consigned to the bin (so far), with everything else seemingly running okay, albeit after a few updates. Mail decided to 'flag' hundreds of emails that were not flagged before, and also told me that hundreds more emails were unread, and after a forced rebuild did the same thing again with different emails, so I had to remove all flags!
My iPhone synced fine (iOS9 seems to store smaller versions of all photos, saving space), but my iPad mini threw a wobbly and refused to sync the photos, telling me that I was massively over-memory even though the pictures were already on the device! In the end I restored the iPad to factory settings and then did a fresh sync, which lost me all my settings and config values, only to find the same thing happening again. After many more hours trying everything I could to get things to work, only after I disabled Find-my-iPhone did everything sync properly (after which I enabled it again), so there is clearly a nasty bug there!
So I did it, spent a full day doing so, but now I have a working and up-to-date Mac OS and device OSs. The Mac OS was free and things could be worse: I could be trying to update Windows!