THanks for help. I wasn't able to follow the commands you posted in. I pasted them into the terminal and got error message which I can't remember. Tried with smaller files too to ensure it wasn't a memory issue but same errors. When you don't know what you are doing and there are no examples shown or anything in the manuals, it is hard to figure out what to do.
In any case, once opened as Root, could delete the files, thereby freeing up 21.8 GB in the Linux partition.
IMO there is something not quite right about an OS which can casually build 10 GB txt files without warning you that something is going on under the hood.
I also still find it strange that I cannot see where the LInux partition is when I am in Linux, only when in Windows. I installed and opened GParted at which point I could see where it has been placed (smallest partition on my smaller-with-little-free-space and older and slower backup HD for some reason!). I don't understand why I cannot see that normally with the Files.
I intuitively like Linux and like Mint. But am definitely struggling with being unable to see what is going on - especially when there are problems. No doubt will learn over time, but it's going to be slow. Meanwhile, am using Linux as a way to reduce computer usage (no longer updating website for example since unable to figure out how to do it) which has its own upsides - life is simpler!!
My son just graduated from German computer college, so when he comes to visit in a couple of months he'll set me straight on all this. I suspect that's the best way for most non-geeks to learn, with live help.
That said, I suggest (there isn't any clear place to suggest, is there?) that Mint development team do much more work on a Manual document that addresses most core issues. The current document is concerned only with installation and even then it doesn't explain anything other than how to install. The installation is experience is much better than years ago, but what then? There remains a lot to be done. Some videos wouldn't be bad showing people basic functions (like how to open a file and get those last lines like you suggested above), but generally these things are clearer and simpler in documents with pictures. Easier to reference and file and quickly look up the info if you are coming back to it after several months gap.
Thanks again. All best. Will keep grinding along in Linux and no doubt slowly, slowly this almost-60 year old will get there!