An opportunity for serious fraud arises where a landlord rents out a property without a mortgage. The fraudsters will attempt to obtain ID in the landlord's name (even going so far as to change their name by deed poll in order to apply for it), and once secured will either try to sell the property or take a large loan against it. They will then disappear with the money, leaving a very difficult and traumatic mess for the other parties to resolve.
There is a very useful free service from the land registry that sends the property owner an alert if certain types of activity take place against a property. We highly recommend that all landlords make use of it, but especially ones whose property is mortgage free: https://propertyalert.landregistry.gov.uk/