Legal Position

Possession - Section 8 and Section 21

Dear Sir/Madam,
I let a property to a mature couple two months ago, and the shop underneath has told me that there are four young girls living there, and lots of elderly men going in and out of the flat.
I have tried to contact the "tenants" but phones are going to voicemail and emails remain unanswered. I've also knocked at the door, but no one answers.
I suspect the property is being used as a brothel and want to issue Section 8 Notice to get them out, but can you kindly advise what sort of proof I will need in order to serve the Section 8 Notice please?
With thanks in advance,
Adam Powell