challenge to agreement

My ex and I drew up a Separation Agreement in DEC 2020 which was honored in NOV 2021 following house sale (63.89 to 36.11 in my favour). Following a mediation session he has decided that as he believes I had an affair that he now wants to undo our agreement and says he will be taking me to court. I am about to buy a house with my new partner and my son lives with us (he is 18/has ASD and is in full time education) The agreement was originally drawn up in my favour so that I could buy a comfortable home for us and to allow for spousal maintenance. My ex was previously a high earner (approx 100k) but since redundancy at the time of the agreement has chosen to work cash in hand and I am not working as I had to give up my job due to stress and anxiety and have tried to return to office work but could not cope due to stress. I am now studying horticulture at college but am limited to the amount of days I can work due to ongoing back issues. We both agreed on an online divorce as originally I was divorcing due to issues of control and a DV incident upon myself by my ex.
Is it likely that the split would be seen as unfair and what should evidence should I be preparing if he does decide to take it through the courts to challenge it?

thanks for any help and advice.