dont have much to advise but from what i know on this statement.... when bring them into court / tribunal, the main thing is you as tenant will have to show proof if you had paid for the rental instead of the owner if they had received any rental fee or not.
Locking the main door is "crime". even if tenant dint pay rental for xx months.. owner still dont have the authority to lock / break in the house etc.. police report can be made on this issue.