Hi James, the following is my understanding of the situation based on my knowledge of English lets. There are a few difference between England and Wales, but I believe the following applies in Wales, too. Best to double check on a site like Shelter Cymru to be sure.
There’s nothing illegal about staying in a property until the landlord has the legal right to repossess it. They only have this right when the have served a valid notice, successfully obtained a court order of possession, the order has expired, and they have then applied for and got a warrant for possession. At this point only bailiffs can evict you. At the point that the court orders you give possession of the property to the landlord, it might be said you are in some way acting againg against the law insofar as you are not acting in line with an order from a court, but in practical terms, not much can be done to make you leave until the warrant for possession is obtained.
One thing to check is whether the landlord’s section 21 notice was served validly. In England at least, a S21 cannot be served validly in the first four months of a tenancy. If this is the case in Wales, that would suggest that the notice is not valid and the landlord would have to start afresh with any eviction proceedings.
Sam