Cohabitating

What constitutes cohabitation. Is staying over at my new partners house sufficient, or do we need to be paying bills etc together and pooling our finances In order to be regarded as cohabiting.

There is no legal definition of cohabitation. It is generally taken to mean living together in a relationship akin to marriage, where parties (usually) live in one properties and share bills and other outgoings together, and have a financial link, ie joint bank account.
If you each maintain properties of your own, receive post at your own properties, pay bills, are registered to vote/pay council tax etc at your seperate properties, and are simply staying over at each other's homes on occasion then it could be argued that you are not cohabiting.

Contrary to popular belief, there are not a set number of nights that mean that a couple is co-habiting. It is usually assessed on the specific facts of the case, and a judge can make a determination, even if the couple deny cohabiting . Anything less than four nights a week would almost certainly be decided at the discretion of the judge.