no tenancy agreement

Tenancy Types and Management

Hello
I am helping a landlady who has a nasty tenant and there are asbo issues,antisocial behaviour,no rent, drug taking etc. She is elderly and suicidal about it all. He took the tenancy agreement off her and won't give it back so she doesn't know the tenancy start date so thereby making a s21 difficult... any suggestions on how to serve it without that information?

the police have done nothing to help but the asbo lady from the council has threatened to close the whole block of 8 down unless she does something about it...

suggestions please!

thanks