This poster doen't seem to understand how tenancy law works. If a Fixed Term tenancy agreement ends, no new tenancy agreement is signed up to and the tenant remains at the property then a Statutory Periodic Tenancy automatically arises. All the terms of that preceding Fixed Term agreement apply, except for how the tenancy may be ended: the tenant can give one month's notice and the landlords can give two months ( if the rent is required to be paid monthly). Nothing ridiculous about it.
ukheman - youmay want to join one of the National Landlord Associations (NLA or RLA) . They can talk you through the appropriate Notice serving and the cost of membership can be put down on your annual tax return.
If the tenant paid you a deposit then you should have complied with the law and registered the deposit with one of three schemes, plus given the tenant the prescribed information from the scheme. If you have not done this you cannot serve a valid s21 Notice.