I suspect that under the conditions outlined here (assuming that all the relevant facts have been revealed- which is a job for a lawyer), the tenants’ right to privacy has probably been breached. The tenants probably have a good case if they wanted to take the case to courts, as a breach of human rights.

I would suggest that the tenants take further steps. I didn’t know that they had a right to change the locks, but I would have thought that they would have to provide the landlord/ agents with a spare set.

I would recommend setting up a small webcam, linked to a computer – it is quite possible that an agent with that sort of track record is capable of entering the house when the tenants are not at home. They need to track this.