Method of Tenant Notice

Tenancy Types and Management

Hi, hope you can help.
If the tenant wants to give notice to end the Tenancy must this notice be in writing (Or can we insist on this) or must we accept a verbal notice? I know a Tenant of ours is planning to leave but don't know when. I have tried to assist by writing to her to explain the process (Notice period, dates, check-out process etc)but have not heard from her.

Obviously without written notice there will be no evidence of when the notice was given or the date they wish to leave (And therefore when we have the right to conduct viewings.

Our notice to her of course must be in writing.