ajobbins:trig42: I think the difference between a squatter and a trespasser is that the squatter has gained 'lawful' possession of the property (ie., they were a tenant, but their tenancy expired, or they did not break and enter). A trespasser (who the Police can arrest) gains unlawful possession.
I suppose you could argue that when he was let in originally by the former tenant, he lawfully gained possession at that point, however if he now leaves the property at any time and returns, he is illegally entering the property. I would guess that as the locks have been changed unlawfully, he is using an illegal means to enter the property - he might as well smash a window to get it. Thoughts?
That's what I would of thought. If there was a sub-tenancy then someone must have some paperwork for it... and if there isn't then he must be there unlawfully. (IANAL)