Tenants wanting to end there tenancy after 3 weeks

Other

My tenants moved in on the 7th March 2011 on a 6 month AST.
Payed deposit which was passed on to DPS and 4 weeks rent payed on the 7th of the of March. Her housing benifit has just been sorted out yat i have not received any further payment as yet.
I got a phone call off her last night saying she has to move out due to family 2 doors away causing trouble with her brother who lives further down the road from her and police was called. She said she had no part in this but wants a quiet life and does not want to be dragged into this trouble.
Does she have to give me notice in writting and how much?
Please advise of anything else i should be mindful of.

Regards

Caza