Mother of tenant moved in without permission

Hi.  We have a standard 6-month AST agreement with two ladies who met at work and thought that sharing our three-bedroom property would work for them.  They have been at the property for over one year, and we have never had any issues with them, in fact, when we needed a plumber to go in and adjust the shower he commented that he has never seen such a well-maintained bathroom. We found out in the last few days that one of the signatories to the agreement installed her mother into the property together with her [the tenant's] son.  Neighbours have also started saying that she has started causing problems, with a lot of shouting and arguments.  I also understand that as she overstayed her holiday in the summer of 2021, she was sacked from her job and has been on benefits since.

I would like to at the very least have her mother out of the property as she has never had permission.  Ideally, I would like to move forward with removing all of that side of the agreement, the argumentative tenant, her mother, and her son.  How can I do that?  Is it just a matter of terminating the agreement, and rolling over into a new agreement in the name of the remaining tenant?  Any help or advice would be great please all.

Thanks

Steve

There are now four people in more than one household in the property, which has created an HMO.
Unless your local authority has an extended licensing scheme, it's probably not one that needs a licence (although you should check on their website).

Unless the property is set us to support the use as an HMO, you need to revert it back to either one household or two people maximum.

The risk of one of two sharers doing what has happened here is why many landlords will not let to a couple who are not in a relationship with each other.