We co-own a patent or application. What can each of us do?

Co-ownership works on a default set of rules that an agreement can change. Start with what you are trying to do.

High complexity - significant legal consequences

A suit must be filed in a court at least as senior as a district court that has jurisdiction. If the defendant counterclaims for revocation, the whole suit goes to the High Court (section 104).
The sections we checked do not say whether one co-owner can sue alone. Take advice from an advocate before filing without the others, and consider having all co-owners join or consent in writing.
Make sure title is recorded: a document not entered in the register generally cannot be accepted as proof of title (section 69(5)).
Agree how legal costs and any damages will be shared before you start.

The honest answer usually needs someone to look at your actual disclosure, your timeline and the prior art. That is a conversation, not an article.