Struggled on, nothing worked. I'm experienced with MSDOS and there was nothing wrong with the paths, the directories and files all existed.
As a last resort, I downloaded a proper file manager which can look at the system disk and conformed that all was correct.
So then I tried a new tack and got myself root access. Everything then went as it should have.
It points to a couple of things for me.

1/ I do not appreciate a file system which locks me, the owner, out of it so that I cannot perform a basic file operation. Even Windows has the grace to ask if you really want to and then steps aside. And it requires a download for me to do it! Nanny state or what?

2/ The error messages given by Linux seem even more opaque than Windows. A proper error message would have said 'you require root access to perform this operation'. Instead I got 'file or directory does not exist' when I knew they both did.

I wonder whether this is an attempt to keep the OS to geeks (no insult intended, I'm one in other systems) or that in the open source filed there's no incentive to improve it.

Thanks for the help, folks.