Re:step children

Hi, My partner is currently going through a divorce. He was married for three years and was co habiting for 9 years previous to that. When he met his ex she had two children. She was not working. The children are now 14 and 16. They have never had any contact with the natural father, although the natural father has tried to make contact with the children. The natural father has never paid any maintenance, but neither has he been asked to by my partners ex. There is no house to sell. My partner and his ex are both in rented accomodation. They have 3 credit cards debts and loans plus overdrafts totalling £15k. My partner has a small pension worth £15k. My partner has just received £4k personal injury compensation claim. His ex received inheritance of £1500 earlier last year, of which my partner has not received anything.
My partner has a daughter from a previous relationship who he sees regularly and pays maintenance for. My partner has not been involved in decision making for the step children. He has continued to see them since the separation. Although since he met me, the step children are not happy to come and see us.

My partners ex is asking for £200 per month "childs funds", half of his pension and half of the personal injury claim. She is not willing to contribute towards the debts. She is not working and never has. My partner is offering to take responsibility for all of the debts, let her have half of his pension and will not ask for half of her inheritance. He is not willing to give her half of his personal injury compensation. He does not want to pay any maintenance towards the step children and would prefer if his ex got this via the natural father.

What are his chances of achieving this? Any advice would be gratefully received!

Stepchildren are only ever considered 'children of the marriage' if there is a housing pot, which in this case there isnt (ie she keeps the house or the larger share because she has 2 children living with her)

As for CM the first port of call is the natural father. He has no financial obligation to these children.

Debts? whose name are they in?
if they are all his then he should retain his pension as to cancel out the debt and then essentially he walks away with nothing.
It would cost her a bomb in legal fees to sort this out and wouldnt be worth it for such small sums. To be honest he is best to just get a clean break order asap, and they both take what they put in.
what stage is the divorce at?
Whats his income level?

My partner has his first solicitors appointment on Monday. This is the start of his divorce. He earns £32k a year. His ex does not work. Thank you for your reply it's very helpful!