I'm not sure about all your laws in the UK, but here, with you having custody and her not able to stay sober you would be able to get what we call "supervised visitation."
Hopefully you can get something like that there. Visitation times are set in concrete, and an 'impartial party' (usually a social worker) supervises the visits at a 'neutral' place. Then when visit is over, the children would come back to you.
It might be worth checking into.
I understand your frustration, and attending some Alanon meetings would be great for you. Unfortunately, those of us that have alcoholics and/or addicts in our lives are very affected by their actions.
Please keep posting and let us know how YOU are doing, we do care very much.
Love and hugs,