Why do you think he doesn't have a leg to stand on? That's a genuine question.What does your client want to achieve? Sounds like a right load of stress and hassle, I would've just moved my things.
It's not part of his flat so he doesn't have a leg to stand on.
It appears to me that the prescriptive easement legislation is quite sound. Did you read that and what did you think?
There have been cases where tenants have challenged landlords on this basis and won apparently.
Anyway, this maybe wasn't the best place to post about this as it seems some are already getting irritated.
Sorry... didn't mean to upset.