Take a minute to look at this from the other side. She doesn’t want to revise at home or flat out refuses to revise at home (two different things imo) but what is she doing with her time at home instead?
If she’s basically completely mentally switched off once she’s home, at this point I wouldn’t want to waste a lot of time trying to change that. It’s annoying and it would be an ideal habit to have for college and adult life that home is a multi-function space, but that’s a whole extra thing to learn so probably not the best use of time for year 11. In that case, figure out at least one space you can get her into where this will feel like a study space. Does she see her tutor somewhere out of your home? Library, classroom, tutor’s house? Depending on what that space is, you may be able to add more time there without paying for more tutoring, and see if she will be more switched on and do some revision alone.
If that looks impossible or highly unlikely, then yep book more tutoring and see what you could schedule. You might find you could slot into gaps in someone’s schedule if you’re up front that you would happily take 15, 20, 30 minutes more days a week so she’s revisiting maths more than the one session a week outside of normal school lessons.
But backing up, if she’s not entirely mentally switched off the whole time she’s at home, would she engage with a YouTube video series? A paid for online recorded tutor? An app or website that makes it a bit more of a game than just doing homework?
It is also worth asking at school what intervention plans are in place for kids who are sitting on that borderline in year 11, because school will want her to pass too. There may or may not be things planned for half-term breaks etc and knowing as much as you can now will help you prep for what will be the right balance of helping her without completely burning her out.
Multi-pronged approach that divides things into smaller, regular pieces will be your best way forward. It means the detriment of having an off day or missing part of the plan is far, far less because you’re not banking everything on one or two big things in the schedule.