"She says she was paying a lot less when she stayed with her partner but I can't believe that the rent and bills was a lot less than £250/ month. Her partner wasn't working for most of the time they were together she said she paid all the bills."
Less than £250/month? Well that's a lie, isn't it? And lying to me would be the first thing we'd be discussing when she got home from work, no ifs and buts. No lies accepted. You're her mother and you love her but being lied to by her calls into question whether she loves you. Tell her that, and that you'll only accept honesty from her, that her lying to you hurts you and she shouldn't try to hide things because she's embarrassed (by her debts, her poor choice of men etc) or avoiding facing things.
"Several months ago after the last bail out, I asked to see a bank statement to see if I could see where her money is going to try and help her budget but she didn't want to show me. I can't keep bailing her out but I don't want her to move back in with him.
"
And that's the next thing to discuss. You've been bailing her out. Just how much debt is she in? She needs to be honest with you. And honest with herself too - to face how much debt she's in and why (your posts imply it's because of her partner). Don't be emotionally blackmailed into glossing over her finances by the threat that she'll go back to him.
And she needs to accept that you cannot bail her out financially again, you just do not have the money, especially when her return meant the loss of the single-person council tax discount. You can help her work out a plan, but you cannot bail her out.
If she's going to be part of your household she needs to contribute, not just benefit. She's an adult - so she needs to adult! That means paying rent, contributing to bills, not being a drain.