From a legal standpoint, and with no judgement at all:
If you can prove that you are completely separated, you'll be okay. That means that you can be living in the same house, but you must live like lodgers do. So pay bills separately, pay rent separately, buy separate food, not have access to each others' bank accounts, separate contents' insurance, separate debts/credit accounts etc.
This will be the type of thing that they are now looking into. If you still behave as a couple, your claim will be invalid, and you'll need to make a joint claim until one of you actually leaves. It's generally better to point out that you've made a mistake yourself and make a plan to pay back any overpayment than to let them discover that you've been claiming at the wrong rate, because they'll be looking back over the period of the claim rather than just how things stand at the moment. So cancelling everything this morning won't help!
If you are living separately and your finances and lifestyle prove this, you've got nothing to worry about.
The rules may seem harsh, but there has to be a way of the government differentiating between people who have actually split up and people who are just desperate so pretend to be split up to get a higher payment, and this is how it is currently done.