Possession of property

Possession - Section 8 and Section 21

Please can anyone help me with the following situation:
My tenant is the sole tenant but has had his girl friend and son living in a flat for most of the time
He provided by text he had left the property and was serving notice with 4 weeks to end the tenancy.
I was already seeking to take possession of the flat in the near future so I subsequently served 2 months notice for which he has failed to pay one month.
Now his girlfriend and son (they have split up) are still saying there and are refusing to vacate the property despite my giving them 2 full months notice from the date that he originally gave.
She is saying despite the fact the is not a legal tenant and that they are in arrears she can stay here.
Can I change the locks when she is absent? I need to refurbish the flat asap .Do I need to serve a section 8 notice against her former partner despite him not being there.