@Simon758
Please correct me if I'm misreading what you've posted about this situation:
1. Guest booked for an unspecified amount of time through Airbnb;
2. You and guest agreed to allow her to stay for a further 2 weeks. Reading between the lines, it seems that this was not done through Airbnb, but a private arrangement, where she gave you the cash.
3. The 2 weeks elapsed, and when you went to clean the room, she was still there, did not give you any further rent, and suggested that you were incorrect about the further length of time she paid for. You did not insist on her vacating the premises at that time. She continued occupying the room without paying for an unspecified time.
4. You made personal contact with her last Saturday. She refused to pay and you simply left her there without any action, other than to suggest she should "probably" move on.
5. You've received a rambling message from her, off platform, basically saying she's not moving out, not paying you, and there's nothing you can do about it.
It seems that you stitched yourself up at point #2. Once you accept a private arrangement, Airbnb will not (and shouldn't) offer you any protection from overstaying. I would think that the only remedies open to you now are through a civil route. I'm unclear as to what UK laws are regarding trespass/squatting/tenant rights, and where this situation falls amongst them.
In my mind you have 2 options at this point:
1. Enter the premises, with a witness (a police officer if at all possible), remove her and her belongings, change the locks and let the fallout begin; or
2. Follow your solicitor's advice and begin the section 8 court action (whatever that might be) without delay.
What a terrible mess.
Best of luck.