change of mind after papers have been signed

hello, please can someone answer this question/s for me? My partners ex agreed to a amicable divorce based on 2 years separation(they've been separated almost 4 years). She even paid for half and signed the statements for the children. This all got sent to the court but she has now changed her mind and refuses to return the Acknowledgement of Service unless we give into her unreasonable demands. We are tempted to do this simply because we are desperate for the divorce. My question is if she does sign and send the papers back agreeing to the divorce, how easy or could she actually then change her mind and stop it or is it a case of once signed it's done and there is no going back?

If you know she has received the petition then you could submit a sworn affidavit to the court (your solicitor can write one up) to that effect and the process will proceed without her signature.... that's what my partner has just had to do...

Thankyou Ephelia. But is your partner's divorce based on unreasonable behavior? We were considering amending the divorce to that. Did you have to get a court bailiff to serve the papers? How long has it taken you, if you don;t mind me asking?