Help needed with my unauthorised tenant?

Hello, I have a situation where I was preparing a property for rent and the builder passed a copy of the keys to the neighbour who was their friend, to allow carpet fitters to attend the property. The neighbour then indicated that their daughter would like to rent the property.

I agreed in principal, subject to satisfactory vetting/financial checks by the letting agent.

The prospective tenant failed the vetting procedure but I then discovered she had been given the keys to the property by the neighbour (her father) and had moved into the property.

They have failed to move out and have paid no rent.

Can I gain access to the property to repossess it? (It has been suggested to me there is a name of this type of tenant and I can enter the premises and change the locks when they are away from the property).

Or do I need to go through the Courts?

I would appreciate some advice please.

Thank you.

Matt