Only caravans have a time fixed of one year in IET regs. Mainly as condition will dictate as to how long before next one.
However in their other publications it is considered that it should be tested no longer than every 10 years and on change of occupant.
As Eric says the IET regs don't have legal status so recommend for domestic properties 10 Years.
Change of tenant would be a good thing, to ensure next one in is taking over a safe place.
The point that counts is your legal defence following an accident/incedent!
Unless your insurence company requires a regular inspection or you rent out through a local authority, or housing association who have their own set times, then it is unfortunatly down to your own risk assessment.
Likewise for fitting of smoke alarms, emergency lights, fire blankets etc.
As a rental property could be considered as a business it has been said that the Electricity @ work act, could apply, this simply states:
"As may be necessary to prevent danger, all systems shall be maintained so as to prevent,
[i]so far as is reasonably practicable[/i],
So IMHO an initial full Periodic Inspection Report from a registered leckie,
with visual checks by yourself for obvious damage at say yearly intervals and at any reletting to a new tenant would show to any court a formal monitoring system is in place.
Personally I carry out a PIR every 3 years with a visual check every 6 Months whilst my wife renews our tenants 'Assured Short term renewable contract' or whatever it's called!