These were very popular when the Rent Act 1977 was in force, and to a lesser extent before the Housing Act 1996, because if you could prove to the Court that you had created a genuine licence, then possession was a lot simpler. A notice for possession of 28 days, after which the occupier became a technical trespasser, subject to Order 24 Proceedings in the County Court. These are quick proceedings for which the only defence is that the occupier was not a technical trespasser, and there was no waiting for 14 days for the bailiff - they could be instructed next day.
There were many variations on the theme.
However, the House of Lords in Street -v- Mountford decided that to create a licence, there had to be some contractual right to deprive the occupant of the use of the property for part of the day, and that right had to be exercised. Their lordships also decided that when you grant a tenancy, you grant "Exclusive Occupation".
That means the tenant can exclude everyone, including the landlord if he so wishes. So if the tenant will not allow you to carry out an inspection, or they change the locks without your consent and will not give you a key, your only recourse is to serve a Section 8 Notice alleging breach of the tenancy and see if the Judge will make a possession order.