How much does she owe you? You could try and take her to court but you woudl need evidence that you had formed a binding contract.
If verbal it is her word against yours and difficult to prove. If shes broke then it may be hard to recover the debt even if the court rules in your favour and makes an order against her.
Speak to the CAB and take the figures and any evidence of an agreement you have with you eg time she was there, payments made, receipts, anything written such a notes referring to her paying rents or towards bil (did you have any agreement about bils or did you just sort them). Also bear in mind any other proof of conduct which would suggest an agreement with rent payable was in place.
They may be able to advise you on whether you could have a decent claim and help you if so.
http://www.citizensadvice.org.uk/
Be careful though as you didn't seem to get rid of her in accordance with the law and you need to be careful of any counterclaim or mitigation. Best to discuss this with CAB as well in case you may be opening a can of worms and incurring costs you won't recover.
May be you just have to put this one down to experience.