Reducing maintenance

Hi,
Overall I have been the rough times of separation and divorce and have remarried.

We married in 2007, separated in 2011 (2 children), divorced 2013. We have never needed to go to court, both of us have used solicitors but just for independent advice, and we did mediate in the early days to sort our co-parenting agreement.

Since then we have sold the house where we lived together and agreed how to split the proceeds (70:30 to her), I have always been the earner and I recognised this with provision of SM until both children were at school full time.

The amount I pay to her now is 20% over and above what the CSA guidelines are.

My thought is that I should just pay the CSA derived CM now that she is engaged, and expecting a child with her partner of 6 years.

They live in a house that is still mortgaged in my name and she pays interest-only from the money I pay to her.

Our eldest child is now at secondary school and we both recognise the increased financial pressure on this transition, clothes, trips etc

However she is always asking me to pay extra costs over and above what I give pay each month. I am happy to contribute to the extra costs but feel I shouldn't need to fund my ex-wife's and new partners living as well - is this unreasonable?

Is there a precedent set for what situational changes make it appropriate to pay CSA derived CM based on my earnings, without the recipients agreement?

TIA
Alex