We've managed to make contact with the right people at the letting agents, and we now have a copy of the tenancy agreement. It runs to about 30 pages and I haven't read every word of it yet. I've read "selectively" and, as expected, there are many conditions under which the tenancy can be terminated. Allowing the property to be used for illegal or immoral purposes, creating a nuisance or annoyance to neighbours, committing an "arrestable" offence in, or in the vicinity of, the property, etc. etc.
There's no doubt the agents CAN take action on a number of counts. Of course they need to check that what we've told them is true, and not just malicious accusations.
Mrs S-H also spotted clauses in the agreement which forbid smoking in the premises without permission, and forbid anyone other than the named tenants from living at the address (having a guest stay over for a night is OK).
The agents have said they'll contact us again today with more info. We await developments with interest.
Garry