Welcome,
So sorry for what brings you here, addiction is very ugly. The system in the UK is much different than here in the states. My husband is from England and the father I am told typically does not have much say. I would maybe suggest writing everything down with dates and such. If you are at any time to try and get custody I think you will need this and proof if possible. Not sure if the school your step-daughter attends will give you written statements about appearance and performance. I would imagine with not much supervision she has probably missed school days.
I am hoping someone who is more familiar with how your system works will see your post.
I will be thinking and prayer for you and your family