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Thanks for your replies. I have copied (most of ?) the contents of my SD card on to my PC without much difficulty. I was going to try a brand new card in the phone and try to format that as additional internal memory. If that is successful, and I succeed in copying the files back from my PC to the new improved phone memory, I can then wipe the original card and use it for whatever.
That, at least, is what I had in mind but I am cautioned by OG's comments about memory size. I notice that the latest Nokia 5 (now 5.3) comes with 64 GB of internal memory which is 4 times what my current phone has. (It's also the same amount as my first PC had back in 2000 - when it cost the thick end of £1000!)
I might just bite the "built in obsolescence" bullet!
Thanks again for the helpful comments.