There are certain rules about cohabitating couples for social welfare purposes, in relation to social welfare they take all the pain but are not allowed to gain by the rules of the tax/revenue/legal system for married couples.
Exactly as Bronte says; Sw treat 'co-habitation' the same as 'marriage' under the SW Act, Revenue don't under their Act! TYhere is no chance that they won't look for your accounts, etc..