Well, I can now report that nail varnish works well! So long as it's the stuff that smells strongly of solvent or something similar.
You will need a bottle of cheap nail varnish remover of some sort to get it off again after etching. White spirit doesn't touch it. My wife gave me an old bottle of varnish that used to belong to one of my children. It's called 'Twilight Teaser'
Basically, it's pink and that's not so good as it's close to the colour of the clean copper and not so easy to see where you've been with it. Black would be better along with a fine artists brush if you want fine lines. The brush that comes with the bottle is OK for large areas, ground planes and so on.
John