Thanks for your patience.
The issue with a separation agreement is that it is not necessarily legally binding, even if signed by both of you. The only final, legal solution is a financial order from the court.
It appears from your question that you have the decree nisi already.
This means that you can apply for the financial order. You have to attend mediation first, as it's a legal requirement. You can find a mediation service with the link here: https://www.familymediationcouncil.org.uk/
Once you have tried this, if you reach agreement you can submit the consent order to the court.If you don't agree, you make the financial application for the court to decide.
Either way, you make the application on the form A, here: https://www.gov.uk/government/publications/form-a-notice-of-intention-to-proceed-with-an-application-for-a-financial-order
There's a really helpful guide to this process in the PDF link here: http://alc.org.uk/uploads/Applying_for_a_financial_order_without_the_help_of_a_lawyer.pdf
I hope that this answers your question?
Thank you for your enquiry today. I am happy to answer follow-up questions - please do get in touch with requests for extra information or further queries and I will do my best to help you.