Ex not Taking her Debt Seriously?

My ex owes me a lot of money. We're still living together but in separate rooms. I'm working on getting her to acknowledge the debt in writing but she doesn't take it seriously. So like with this month's payment, she said she won't be able to afford it yet I've seen her pay for two Theory tests, multiple takeaways and also presents for her new boyfriend...

She also says she will only be able to afford £50 a month until January rather than the £100 I have requested, but feel like that shouldn't be my problem when her current spending is the way it is.

Any advice on what I can do?
Ex not Taking her Debt Seriously?
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