With the proposed changes to landlord and tenant law, tenants may be inclined to adopt a more casual attitude to events as their lease term ends, even if they have given notice that they will not renew.
Whatever else is done, tenants should be aware that under the current legislation (The Distress for Rent Act 1737!), a landlord can charge a tenant who is occupying premises illegally a rent of double the existing rent. Accordingly, if you give notice that you will not be renewing a lease, make sure you get out in time. Delay could be expensive!