This happened to me. I phoned each direct debit to tell them that I had told my exh I was leaving and he emptied the account and stopped my access to any money. As I was honest with them they offered to let me pay a different way and waive all charges. I mentioned "abusive relationship" and it seemed like they have a policy to follow.
Then I went into the bank and asked for a print out of the account spending as far back as they could go, so I could show what was normal etc. and had a record of when he emptied the account.
Go to a completely new bank and open an account (you can do it with a pound) and then phone child benefit and have it sent there.
The minute you can, report him to CMS and start the claim. They will go back 3 months to when you separate.
As hells said, if you have family you can go to, go now, and this becomes the date you separate.
You can have some legal advice for free, and then if you need to run up a solicitor bill they will let you pay by instalments afterwards.
Don't worry - you have done THE right thing, but the next year will be hard. But DON'T GO BACK!