That all sounds really weird, and out of step with the rest of the sector.
How can a meeting to establish a performance management plan be informal? When you say a salary band do you mean a spine point or a grade? (If you'd rather take to private feel free to PM me).
I think the main things I suggest would be (1) make sure you have a written trail of everything, if they don't give you minutes send an email summarising what you took from the meeting, (2) try to bring someone to the meeting. Basic stuff but useful nevertheless.
Also you can refuse to agree to anything before you have had a chance to consult with your union rep.
Good luck anyway.