Do we both have to agree to be separated?

From the desk of Louise P. Campbell Q.C.:

No.  It only takes one person to say the marriage is over, and to leave.  It is possible to be separated within the same house, but that can become difficult, emotionally, and practically, especially if one person says we are separated, and the other person says not, so it is best to either leave, or get the other person to leave.  Of course, if there are children involved, then their stability,  the family economics and the emotional wellbeing of the children are significant considerations regarding who should leave.  It is also possible to get a court order forcing someone to leave, if that person is putting the others in danger.

However, you do have to agree on the terms of the separation or again a court order will have to be applied for and granted.