I have read this thread with a heavy heart. There must be something that can be done legally for your stepdaughter. I have studied family law in the US, which I am sure is different than the UK but many of our legal premises are based on UK law. I would suggest speaking with an attorney. I know in the US your H would be able to petition the court for more visitation and/or custody. The court would appoint a Guardian ad litem. Investigations would be done. You could even force the mother to have a psych eval/drug test. Again, I don't know if that is possible in UK. For the sake of your stepdaughter, I think speaking to counsel would help determine what your options are.
As to your stepdaughter defending her mother: I am the ACOA of an ACOA. My mother didn't drink but all characteristics of an alcoholic. Despite horrible things that went on when I was young, I would defend my mother to the end. She was all I had. She was all I really knew. I was used to being in that crazy situation. Change was NOT my friend as a child. If you told me that you were taking me away from her, I would have refused. My codependency had long developed by the time I was 12. I would never have turned on her. I was deeply afraid of her and even more afraid of being abandoned by her and being left all alone. Your stepdaughter will not turn on her mother. Solution? IDK - I think she needs to go to therapy - even if its only when she visits you. She needs to hear from an objective 3rd party who is not threatening to upset her world. She needs to be educated on codependency and ACOA and alcoholism.
I am now 44 yrs old. I have made poor life choices in terms of my relationships all my life. I basically become involved with men who were like my mother. Alcoholics, narcissists, codependents... AND have just this past week realized what was going on with me. I hope you are able to get help for your stepdaughter before she has serious life choices to make.
I wish you all the best.