My partner is in a messy break up with his ex partner, and she is making aligations about me that is very untrue and hurtfull how can I legaly stop her

The question is WHO is she making these allegations to and what damages has it caused you?

I am a very poorly lady and have a liver disease which is not alcohol related it's a virus called PBC, she is telling friend and her children that I am a alcohlic and a drug user and dealer and it is getting back to me and my children which is causing me a high level of stress that I cannot afford to have I am on a great amount of medication and I am in and out of hospital and I am awaiting a liver transplant, it is upsetting my two teenage children which again is not very nice for them to have to hear that from their friends etc

Is your partner in a legal battle with this person? They might be able to help you get a restraining order against the other person for conduct. Restraining/conduct orders are issued all the time.

what solicitor deals with slander
I assume you are in the UK.Any solicitor has right to deal with slander, Thus this link may be helpful for you
Defamation: Find Slander / Defamation Solicitors in UK / London (http://www.contactlaw.co.uk/defamation/?cladg=4526&gclid=clqa5j2iuamcfqi03wodlck5ea)
Contact Law has a network of expert defamation solicitors located throughout the UK who can advise you on any defamation matter, whether it be in regard to libel and slander claims, or reputation management and privacy issues.

There is no charge to you for using the Contact Law service.