I had the same problem a few years ago and it was a fooking nightmare. If she has a tenancy agreement then you will have to serve her the proper notice. If you evict her without serving notice etc, she can do you for unlawful eviction and you will have to pay her housing costs etc.
I would contact your local housing advice centre, they are for landlords as well as tenants, and are very helpful.
We got a ccj against her to try and get the unpaid rent back, after 2 years of trying we gave up, as trying to sell chocolate fire guards would be easier!!
She even set up credit in our names, the sl@g!
I believe in what goes around comes around, so she will get hers eventually!!! 😈
I feel for you mate.
Good luck