As a Landlord you should have Landlord Insurance which will require you, (as does the Landlord and Tenant Act 1995) to ensure that the Electrical Installation and the electrical equipment is safe and does not carry a risk of death or injury to persons or to property.
The only way to achieve this is to have an Electrical Installation Condition Report (EICR) carried out at regular intervals to ensure the safety of the installation.
The interval between reports will depend on
The frequency of occupation.
Change of Tenancy
The age of the wiring
The use of the Installation.
All electrical installations once installed cannot be ignored because over time and with use they will deteriorate.
It is also possible that outgoing tenants could have damaged the installation.