Hello, I am a solicitor with 20 years experience. I will try to help you with this.
Firstly I would say ask you solicitor who knows more about this than I can on this forum.
But in general you do not need a possession order. You can summarily evict them as long as you do not use force. Ie you can change the locks when they are out of the building.
OK once you have changed the locks then arrange a time to return the property to them. If necessary you can make it a condition that a police officer be present (if you can get one to agree to it) or that it be dealt with by a mutually accpetable third party or with a third party in attendance.
But, I emphasise you should check with your lawyer who is already acting.