As far as I know if you move your partner in after I think six months she is untitled to a percentage of your property.
I think the person has to be able to show evidence of having made a contribution to the running of the house - even paying household bills will suffice.
recent legislation signals change for cohabiting couples:
he/she is able to seek redress if the relationship is more than 3 years
(two years if there are children)