Alas...
Many thanks to Charles & Z. I have gone through all the ideas here but nothing has worked. Finally, I deleted the Cake files & re-installed them, but this did not work. I got the Cake Wallet files from a Persistent back-up and checked the seed phrase, which is fine. The password also works fine. There is a file called 'transactions', but I can't read it in the text editor.
There are still only the two 'official' nodes, plus the Seth node I added, but only the top node shows green, and this drops in and out of conection continually. I fear the only conclusion is that the send address was wrong... (I pasted it so I'm not sure how - maybe a space at the end, or something?)
All very distressing. But, since the nodes are not loading, I wonder if it's a program glitch. Maybe I can load Cake in Windows and restore the wallet there? Is that possible?
Thanks again for your help gents (I presume),
Cheers,
Chris
In case what has been suggested above does not work, I would just go ahead and try to install Cake on another platform. Say your phone for example, and try to restore it there. Just in case there is a problem with the desktop/linux version.
You beat me to it Omega! That's my next plan (thanks for confirming).

Cheers,
Chris