Hi, thank you for your question. If the order makes no provision for spousal maintenance to stop upon your cohabitation with a new partner, then your ex-husband is breaching the order and he will be liable for the arrears that he is accumulating.
I would suggest that you make an urgent application to court for enforcement of the order. This can be done using form D50K and a £155 court fee. The application is to court for enforcement by such method of enforcement the court may consider appropriate. This can be as an attachment to earnings, charging order or any other method to settle the issue.
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