Read up on some negotiation techniques.
Zopa is the zone of possible agreement, you both need to know your walk away position, and then find if there is any over lap. In the over lap is where you can both agree.
So you want 1k a month. He offers 200. He will walk at 1k and take his chances in court. You will walk at 200 and take your chances in court . But he won't walk at 600. And you won't walk at 400. As such your zone of possible agreement is within 400 to 600. The numbers are made up obviously.
But you both need to know your absolute walk away number. So you both need to be open to communicating with each other, or the mediator, to find your zone of possible agreement. And you need to consider what else can be thrown in.
For example would either of you give up the value of the equity for increased or decreased monthly payments over a number of years ie ten or twenty? Would you give up pension for an increased lump sum? If so how much will that be? How much would you need to buy a small apartment out right? What about shared ownership where you have minimal rent after? What lump sum do you need for that?
It's basically a negotiation. Think of what your walk away position is. Think of all the things you would accept to sweeten the deal. Think of what you can offer him.for example. If you give me 60k lump sum, (30house equity, 30 other, he just increases current mortgage by 60k) could you buy a shared ownership property, and have a reduction in payments over twenty years? To give you security of a home instead of renting?
It's not a demand, or a blinking competition, it's a negotiation, so understanding what you will and won't accept, what you need, and what he will or won't accept and what he needs will be critical to its success.
Because if you both go in there with no room for movement, then it will go to court, and they will decide. And that could come down to who has the best barrister.