Joining a union probably won't help now - they won't represent or provide legal cover to people who only join when they already have a problem. It's like insurance... taking out a policy after the fire has started doesn't cut it.
ACAS can be indifferent, and also they can give you guidance on the law, but not legal advice (just as well because I often see them getting it wrong).
Honestly, an employment lawyer is probably not a worthwhile investment right now. They can advise you, but nothing more, and I can tell you now what that advice should be.
Unless you have evidence then there will be very little that you can expect if your employer won't do anything - at the very least, a diary of events is necessary, but obviously that is your word against someone elses, so witnesses are better. It's not an either or - you need a diary and you need actual evidence. I wouldn't ever suggest recording people secretly because that might not be entirely lawful, but if you really have a very bad memeory and need a recording just to help you remember things.... I trust I make myself clear on that???
You will need to take this formal, so you need your grievance procedure and you need to follow it. The union might let you get advice from a local rep for this process - it really is up to them. My union would, but bear in mind, as I said, the problem has already been ongoing, so you can't expect anything more than that.
When you have exhausted the grievance procedure and nothing has changed, you might have a case of something, but bullying cases are notoriously hard to prove; plus you would probably have to resign and bring a claim of constructive unfair dismissal which are the very hardest to win - without actual evidence it wouldn't be something I'd recommend.
Or you could look for another job. Probably the fastest and least stressful option that guarantees you still have a job at the end of it.
That sounds depressing. Sorry but in my experience it often is and I won't sugar coat it for you. If you could even construct a case it will be a hard and lengthy road, very stressful, and even if your employer supported your grievance, the outcome may be a lot less than you would want. All too often victims are shuffled off to other jobs, or compliants not upheld. It's really rare that the bullies get what ought to be coming to them. Your employer might be one of the few who don't act like that, but the fact you are here suggests that isn't very likely - your bully has got away with this for years and blaming the new manager isn't really cutting it - they still got away with it for years. A good manager / employer stops bullying, they don't just mitigate it.