Ianal, but I'm fairly sure that the rules around disciplinary process are guidelines that are not of themselves legal requirements, but rather a way to ensure that if it went to an industrial tribunal, they would not be found guilty of constructive dismissal.
As I understand it, for any legal implications to happen, either you have to be fired, or you quit and in either case sue for constructive dismissal - yes, you can claim this even if you quit, if your claim is that they breached your contract, or create an inhospitable working environment.
Anyway, all I'm saying is that you shouldn't charge in saying "I'm entitled to this and that", because whilst your going through internal process, they are only bound by thier own guidelines, and even then only if they actually enforce them, so your not legally entitled to anything. That's not to say you can't bend them over a barrel at tribunal later if they haven't followed the guidelines properly, but for now, you should be careful.
Take a tape recorder (as well as your union rep). You must ask their permisson to record, and they are entitled to refuse, but it doesn't hurt to send the message that you are taking it seriously, and if they allow it, it may be handy later.
Take notes. You said in another thread that your manager is aggressive towards you. I imagine that someone from hr will lead this, but if not, and she is aggressive, or swears, make a note of it.
Finally, don't assume they are just "out to get you". Maybe they are, but keep an open mind there may really be performance issues at your end that you don't realise. If they really are doing things properly, this process is really about finding out what is preventing you performing and helping you with that problem. Listen to what they say and consider it.