Thank you to everyone who has commented.
I am an experienced landlord, and no longer use letting agents as they rarely saved me any difficulty. In this situation they would still have had to contact me, but would also have charged me for the privilege.
I am quite laid back. There is no point in any sort of knee jerk reaction to someone else being in the property. Even if I knew conclusively that the tenant was subletting the whole property ( which I really don’t think is the case) I would still have to go through due court process to evict, which could take months.
I think in this case the tenant has made things worse by not speaking to me. If his friend/ cousin wants to live there and has the right to reside in the UK I would be happy to facilitate this. He has been a good and conscientious tenant otherwise.
I also have no objection to some caged pets, but again I need to be made aware of them and would need to check that there were no issues with overloaded electrics, odour or anything else detrimental to the property.
I issue a specimen Tenancy Agreement via email several days before the meeting to sign the paper agreement so that it can be read and understood, but it’s clearly not happened here.
I have arranged to go and see the tenant on his return. I am pretty confident that all concerns can be resolved. All of this drama could have been avoided if he had just spoken to me beforehand, and I will be stressing that he communicates more effectively in future.
If it can’t all be resolved, or there is more to the situation than is already apparent, I will have to consider if other action is necessary.