Can I set an expiry date for somebody I am adding onto the staff bank to ensure their documents are re-checked?

You can do this via the Digital Safe box located in the candidates profile. Each document can have a different expiry date if necessary, and you can set this when you upload the documents.

When the document is due to expire in exactly 30 days, there will be an alert on Passport Plus so that you can follow up with the candidate about this.