I've been in a similar situation a year or so back.
Bullied for 4 years by 2 people who came in after me, but I kept my head down and tried to ignore it.
Eventually one of them did something that ACAS described as gross misconduct (due to undermining me and bringing the company into disrepute) and effected some of our service providers.
I called my line managers in for a meeting. They were initially quite shocked at what had been done, and said they would retract what had been done and say they had to apologise to me and the service provider.
Two weeks later when I asked what was happening, they said they'd decided it wasn't as bad as all that and nothing was happening.
So I put a grievance in against both of the 2 people. I had nearly 30 pages of incidents, although a lot were unwitnessed or unprovable.
The line managers kept telling me one thing to my face and then doing another. At one point I had an email conversation which went along the lines of:
"LM: We will never do anything about X and their payment without consulting you first.
Me: Why has X just emailed and said that you spoke to them?
LM: I didn't speak to them.
Me: Why do they think you have? Have they got the wrong person. I'll tell them they're wrong and they don't need to worry.
LM: Oh, well I might have had a little conversation but it was nothing about the payment.
Me: Why are they saying you talked about the payment then?
LM: Well I did mention it I suppose...."
And that was typical.
They decided to split the grievance into two and do them separately, which I did query as their bullying was very much done together, with them backing each other up.
They brought in an external HR company. So first one was done by the company. So the day before they sent the initial report over the HR company was emailing me, and they told me that their report was supporting my grievance. Direct words, no possibility of misinterpretation.
The report went in, and no one apparently other than one of my line managers had seen it for the first two weeks. But one of the bullies started telling people it had been dismissed.
Six weeks later, I receive a report that ignores all witnessed events and says it isn't upheld due to no evidence, and a week later I receive another report for the other bully saying it's also dismissed and they've only talked to the two bullies as they didn't need to talk to me at all. (they didn't talk about the other bullying at all in the first one)
I went off sick (and the bullies and my LMs went round lying their heads off. Then got a new job and resigned.
However I wasn't the first person on their radar for bullying, and people at the firm started putting 2 and 2 together and noticing that anyone that had worked with them had left fairly quickly within a few months, and began to realise that the small things they had seen were actually part of a bigger picture. So they had a pretty uncomfortable time while they tried to play the victim, and no one believed them. One of them is still there and I know no one still wants to work with them, nor trusts them an inch.
One of the LMs resigned shortly afterwards and also left, I suspect because he knew that the report had been altered (not the one who it was sent to) and didn't feel he could speak up, but didn't want to be involved if there was fall out. The other one lasted about another 6 months trying to pretend that they'd done nothing wrong before there was a reshuffle and they went, I suspect with a bit of encouragement.
I'd say don't bother with the grievance. Go off on sick, get a new job and resign. Only reason for doing the grievance is to claim constructive dismissal, because you won't get anything else out of it.
I'd like to see far more legal standing for ACAS. There's a lot of guidelines like "they should speak to witnesses", but if they don't there isn't something set in stone. Or "they should do it within a reasonable timeframe" - mine was 4 months before they'd even decided what to do. ACAS said when I asked them that they didn't think 4 months was reasonable, but a tribunal might decide differently.