I would just like to highlight that large letting agencies do not make any checks on their clients right to let a property. This can and does result result in tenants being given invalid insecure tenancies.
The practice of not verifying the ownership of the property and absence of any letting restrictions I have challenged directly with the Property Ombudsman. His decision is that Lettings agents are not obliged under the code of practice to carry out the £3 instant check of Land Registry records that would instantly prevent fraud and stop some illegal lettings.
Until the code of practice becomes more useful than a chocolate teapot, all tenants should carry out their own checks.