"Trespass while a possession order was in place" to give it a full title as trespass alone is not an offence.
"Trespass" becomes "Aggravated Trespass" and a criminal offence if the trespassers refuse to leave or cause any damage.
As far as I can tell for an offence of aggravated trespass there is no requirement for a possession order and a possession order in itself does not automatically bring about an offence of aggravated trespass.
Though obviously a possession order is meaningful evidence that the trespass was un-wanted and can be proof of notice.
The normal offence used to be breach of the peace on refusing to vacate but in this case the plea was against "trespass while a possession order was in place" which I can't find any law or act referring to??? Perhaps its just poor media reporting, I'll have to wait until the court records get processed.