Thanks for this. I didn't know ACAS are to do the deductions. They rang again last night, saying that the employer didn't quite understand what I asked for. Basically they want to deduct some of it for tax and stuff, themselves, rather than offer a gross amount and leave ACAS to sort the rest of it. What I asked was to list these deductions because I wasn't sure how it worked and I wanted to see it in detail. What's being disputed is how long I was employed with them - they think my last day was when I reported the sickness, so I just wanted to see how they worked it out. They responded to ACAS in email to that, saying that "it's my duty to report my earnings to HMRC". The ACAS advisor wasn't sure what this has to do with anything. ("They sent me an email, and I don't think they understood...." - whaaaat? They keep sending random emails with weird interpretations of what was being said? Noooo waaay, these employers aren't known to do that hahaha....)
Anyway, today is the last possible day of early conciliation and I still haven't seen the draft of that agreement. I think they are waiting until the very last minute so I will either have to accept or decline with no time to request any change at all. I don't think they understand that they are exposing themselves to the risk of a tribunal claim, and I don't think they are getting any advice from anyone, or if they do they certainly don't seem to take it!
I don't know yet what this agreement will say, but I expect to receive it today. If they do send it, depending on how I feel by what is being written, I might accept it because I just want to move on and enjoy my new job. If they don't send it in time I will have no choice but take them to a tribunal

I really want to put this whole hellish workplace behind me but I think they are being vengeful because I took ill when they were on holiday and this is how I'm supposed to pay back. Except that I didn't breach the contract/employment law, they did.