Thank you XxWhiteDovexX and jodigrind11 for your replies. I can already see he's trying to keep me in the situation, telling me he still loves me and considers us married; he kept sending me messages asking if I'm seeing someone else, he needs to hear from me at least once a day to know that I still care. I told him I will speak to him when he's clean and I know it's him.
One problem is we have a house, I've been told I should get the locks changed but I don't think I'm legally permitted to do this. He has come home on one occasion when I wasn't here, and I found evidence that he used. He clearly still views this house as a safety net.