Dave Liquorice wrote
Rod Speed wrote

Compulsory voting would require the addition of a "positive
abstention", though - a "None of the above" option

No, that is always available by just not bothering
to tick any box on the ballot paper etc.

No mark on a ballot paper is just asking for some one to mark it.
OK shouldn't happen but ...

Trivially avoidable by ticking them all.

With some of the complicated proportinal representation systems out
there I wouldn't be surprised to find that a valid vote in some cases

Nope, it isnt anywhere.

Is a tick a valid vote anayway?

It is in plenty of jurisdictions.

The instructions tell you to use a cross and shows a little
image of one just in case you don't know what a cross is.

That varys with the jurisdiction.