Good Lawyer Guide's response

Although you don’t have to have the tenancy in writing for your agreement to be legally valid, it is always wise to set out at least the basic terms in writing and for you both to sign it. You should include the period of the tenancy agreed, the rent to be paid and so on.

If a dispute was ever to arise, resolving any issues will be much easier to sort out if your agreement is in writing from the outset. There are standard forms for basic tenancies available which you will find useful. A written agreement will also serve as a useful record and reminder of what was initially agreed in case you forget.