Changing locks and keys

Other

I am a landlord on a house, where the tenants are 4 adults (all related). This is under an assured shorthold tenancy agreement.

I recently went round to check on some repairs which had been done and as i was leaving the tenants stated they were going to get the locks changed as they were not working properly. I told them that I would get the lock changed and would provide them with a set number of keys. I have since notified them of the date i will change the locks but have not heard back from them at all, which is strange as they usually reply.

is there any reason i should be worried about their behaviour? I suspect there are other people living in the house, though cannot prove it (i give them notice before i turn up). Also, what if they go ahead and change the locks by themselves? What is my legal position here and has anyone come across this before? I had assumed the type of tenancy agreement prevented them from assuming any squatter rights correct?