What defines cohabitation?

I''d be interested in any replies to this question, although I doubt that we will ever be able to push this matter legally for the sake of my step children.
My husband has a mesher order on the property he owned with his ex, and is still on the mortgage. One of the clauses is cohabitation. She has told us she is in a serious relationship - a step in the right direction as she moved her last boyfriend in without a word, although it didn''t last long. The kids are now telling us he stays over 4 or 5 nights a week, his kids visit him at her house, and he contributes to the food bill.
Is this cohabitation in all but name?

There is no legal definition of co-habitation but it is generally accepted that an intimate relationship is a cohabitation one where the parties live in the same house, share household expenses, socialise together, and are seen as a "couple" by their social circle, friends and family.

However, if this man isn''t staying at the house every night then presumably he has his own house, and spends some nights there each week? They could reasonably claim that they are not cohabitating because he is retaining his own house and spends a number of nights there each week.

A give-away would be if he is in the electoral roll at your partner''s ex''s home address.

But it is very difficult to prove cohabitation, and usually when it is a trigger in a Mesher, the trigger isn''t usually reached until there has been a continual period of cohabitation for at least 6 months.

This is something that concerns me, my stbx says he does not live with OW but does not have his own home, he instead stays with friends of hers (so he says). However they socialise together,they go on holidays together and I am sure they share meals etc at her house, he has even told my daughter he is giving her daughter 150 pounds a month towards uni. I honeastly don''t mind where he lives but he is telling me he cannot afford to pay half towards our mortgage but if he has no rent and no bills for a home where is his money going?