A few angles -
Lay down my 4 concrete justifications for more pay first (line managing, responsible for lab resource, representing company to outside partners, losing home-working) and push back that I'd like a figure starting with a 5 not a 4.
Then point out that 2 of my colleagues with the same title as me and same age etc, were on 5xk and 6xk recently before moving on. So I was looking at this sort of figure for my next move regardless of increased responsibilities.
Then if they can't go beyond that, I'm going to propose going 4 days a week for that pay. Which might be a hard sell but realistically, I'll be almost as productive on 4 days so it's a good way to make their numbers work for me.
It's a little bit of a tricky conversation and might be an email back and forth process, or hopefully just an in-person conversation tomorrow at the office. I have to stick to my guns a bit as it WILL be more work/stress. I was hoping for 62k, I'm on 39 now and they offered 44. So a bit of a gulf between my expectations and theirs.
I asked the hiring manager his opinion and he said there might be a 6 month sort of "probation" after which we might modify things. But I'd want that in writing e.g. "After passing probation with these criteria, I will move to 4 days per week at no pay reduction". His advice was think of a 300-400 per month take-home increase and whether than would satisfy me.
I'm prepared to walk away and I'll try to hold onto that rather than sell myself short.