Single occupancy AST installs wife and 2 children?

I don't think there's a lot you can actually do here.

Leting on an AST gives the tenant the right to occupy the proeprty as their home. It is difficult for landlords to put (or manage) restrictions on occupation. There's no legal reason not to advertise no children or smokers, but if a tenant is in breach there's not a lot you can do until the tenancy comes to an end naturally.

From your point of view I'd let the tenant get on with his life, with the exception of dealing with nuiance towards other neighbours - but in practice this is always difficult to deal with.

We manage a few blocks of flats here in my agency and we get similar issues frequently. That's why my own portfolio does not include any flats!

We specify in our tenancy the maximum number of tenants - this is particularly relevant for sharers, where additional or selective licencing may apply.

In addition, have you or your agent requested the relevant documents to ensure right to rent requirements are being complied with, given that is now her defacto main residence.

Whatever your tenant may claim, moving his family in so soon suggests he intended this when he took the tenancy. This won't be easy to resolve and you should expect to look for a formal route.
If the wife isn't a temporary guest then you need to have her checked for Right to Rent - and this may tell you if there's a time limit on her visa.
You should also check your tenancy agreement and any mortgage conditions, which might give you grounds for legal action e.g. all adults must be on the TA, or if wife applied for visa before the tenancy then it begins to provide evidence the tenant lied to gain the tenancy.
I'd ask the downstairs neighbour to formalise the complaint and to substantiate this with dates & times of noise as evidence for action you may need to take in future.

When you let a 2 bed flat it has to be on the assumption that the tenant will wish to use both bedrooms to accommodate people. Why do you think that the building is unsuitable for children? I would refuse to engage with the tenant downstairs. If they wish to complain to the Council they can, but if its for regular family noise he won't get anywhere.

If your tenant is paying rent on time and keeping the flat in good condition, what more do you want ?ask him to be understanding and not disturb the neighbours.
The man downstairs(the moaner ) if he has a problem should report his complaints to the clowncil.
If you come down hard on your tenant for no good, fair reason he may decide to not pay rent and you would then have a long and miserable fight on your hands through the Courts to remove him from the property.

Reply to the comment left by David at 21/02/2023 - 12:38
When I let a two bed flat it is on the assumption that the tenant will not breach the AST. My AST states that only those named on the tenancy can occupy the property.

Given that the resident is from overseas what checks have you done to ensure that the tenant is legally entitled to rent in the UK, and what checks have you done to ensure that the presumed wife and children also have the right to rent in the UK?

Australians have a maximum 6 month stay as VISITORS to the UK anything else would require a visa in their passport. Contact your local police and explain that you suspect you have overstayers in your property after the 6 month time elapses. Get the details of the wife kids etc from your tenant. If he refuses then that increases your level of suspicion to police. Failing that crimestoppers or a call report to Border Force. This should be low hanging fruit for them.

Reply to the comment left by Steve Riley-Snelling at 21/02/2023 - 14:19
I'd be very surprised if that requirement would be enforced by a court as it is a clear breach of the right to family life, prohibiting a tenant from co-habiting with a person met after the start of the tenancy.

Reply to the comment left by Steve Riley-Snelling at 21/02/2023 - 14:19
As said above, likely unenforceable.