When does 2 years seperation start?

[Cambam2010]

Can anyone confirm when you are considered to be legally seperated from in order to divorce using the 2 years seperation requirement (if that sentence makes sense...)?

My STBXH and I stopped being together in Feb 2012 but circumstances meant that we both resided under the same roof until our house was sold in Sept 2012. During that time we still had shared finances as it was easier for all things house related but we were sleeping in seperate rooms and STBXH was involved in other relationships.

We have a 2yo together and I am not keen to rock the boat as far as visitation and finances are concerned as we have sorted this out between us quite amicably but I am keen to break away from him that little bit more by finalising things with a divorce.

thanks