Do you need to provide evidence with the grievance letter or does that come later?
The earlier you provide it the sooner they will be able to consider it, so I’d provide it all upfront to avoid any delay.
What evidence and how should it be structured?
Depends entirely what evidence there is, and what allegation you are making. Could be emails or other documents.
how do you avoid rambling?
Edit edit edit. Make sure you’re not repeating yourself and that there is a clear structure to your grievance, ideally with headings and points underneath in support.
Do you tell them everything or just some key, make them think "oh fuck" things?
Include relevant things. Don’t do the classic thing of throwing in everything you’ve ever been slightly peeved about just to make your grievance ‘bigger’, because then it detracts from the actual meaty stuff and gives the impression of whiny mud-slinging.
Is it them who needs to evidence they haven't discriminated, or me who has to evidence that they have - DH and I disagree, he's a private sector manager, I'm in the public sector
You need to evidence that they might have discriminated, then they need to provide evidence that the situation you describe was not discrimination. So for example if you’d been treated less favourably because of pregnancy, you’d need to provide facts evidencing the alleged less-favourable treatment, from which a tribunal could consider it to have been discrimination, and then your employer would need to prove why the facts you describe were not discrimination and provide sufficient explanation,
Do I need to source witnesses? Are witnesses needed at all? Do they get witnesses? Is there a need for witnesses to be believed?
Depends whether there were incidents which were witnessed. You could either ask witnesses yourself or could just point out that so-and-so was present at the time, and ask your employer to seek evidence from them. Not sure what you mean by is there a need for witnesses to be believed? Obviously if they are not believed your grievance is less likely to be upheld, but unless there is reason to believe they are lying your employer should generally believe witnesses.
Do they need to see my evidence or can I just say what evidence I have? Do they need copies of emails?
They will need to see it. You could direct them to where it is but I suggest you provide it.
What if witnesses refuse to participate?
Not an awful lot you can do about that really.
How should I lay out my evidence? It is currently pages long!
Be concise, structure it well and logically, use headings, usually various allegations with supporting evidence outlined underneath. Make it as easy as possible for someone with no prior knowledge of the circumstances to comprehend. Remember it is being read internally first but might be read wider later on, if you take it further.
What if its not upheld sad. I want to stay in my job but if it's not upheld and then the appeal is not upheld, I don't know what I'd do.
You can either stay as you are, leave your job and/or bring a tribunal claim. Or a settlement might be negotiated.
Can an employment tribunal claim be dated from the grievance decision date as I read online that you should go through the grievance process first which is 4 blooming stages and will take ages.
Not sure what you mean? A tribunal claim is dated from the date you put it in? You would normally be expected to make all reasonable attempts to resolve it internally first, which usually means following the grievance process. Otherwise your employer could claim your legal action was premature and they weren’t given reasonable opportunity to put things right.