The best way to get rid then is with notice, she has been with you for six months so you can dismiss her as she is not 'suitable' for the post, you do not have to give rhyme or reason. Write to her saying that the company is considering terminating her employment and include a date and time for her to meet with you. If she doesn't make the meeting set another date and say that if she fails to turn up the meeting may take place without her and she would then be notified of the outcome.
At the meeting tell her that you feel she is not suitable for the job (don't go into detail and don't let her argue with you) and that you are giving her one weeks notice (if you don't want her to come to work you will have to give pay in leiu of notice, if she walks she loses her notice pay) also say that you will advance any unpaid holiday pay with her last pay slip.
Do not get into a discussion about this, if you are worried about a tribunal don't be, you are well within your rights to do this and even if she brought a claim about discrimination you obviously have information to back up the fact that she lost you a contract and she is unsuitable. She might be shouting and screaming about a tribunal but she will also know that she has caused problems for you so probably won't carry it through.
You need to do this immediately as if she goes on the sick you won't be able to dismiss her and it will make things drag on and become more complicated.
Fox