Going back to what you said about being scared of joining the union because you'd be seen as a trouble maker....
Well, firstly, they don't have to even know about it. You can pay by direct debit and have meetings and communications with your union rep out of work time, etc.
They don't have to write your letters but can certainly advise on the best wording, etc.
It's a bit like insurance - you hope you'll never need it but, when you do need it, you're damned glad you've got it.
Hopefully it will all be sorted amicably but, not wanting to be doom merchant here, if it does go horribly, you'll need more than just advice from Acas. Sometimes you get a boss who is just a bit of an arse, or an issue escalates into something much bigger. And then you're glad that you've got a union and all its huge resources, skills and experience behind you. Plus they have legal teams behind them. And support networks. And financial assistance if you need it.
All in all, you get a lot more than if you went to a no-win, no-fee lawyer, or just tried to cope with it on your own. They will have dealt with so many cases like your own.
Please, at least talk to them!