Hi,
The landlord does have a legal obligation to ensure all of the electrical systems that they provide in the property are safe to the best of their knowledge. This means both the wiring and the accessories. So both PAT and PIR should be done as they are recognised as a method of providing proof the landlord does have a regular regime of safety checks.
The time period is a mute point. In rented properties the PIR is between 3 and 5 years depending on the findings of the competent person carrying out the inspection and tests. If there are some low/on the limit readings the inspector could suggest a lesser interval.
A PAT in my opinion is even more important. Lets be frank people in rented accomodation or even more so in holiday lets do not treat the accessories well and i have no pangs of guilt stating all accessories should be PAT tested once a year minimum, whether they are CAT 1 2 OR 3 devices.
The cost of a PAT is relatively low, i tend to compare it with having a carpet cleaned about £70 on average for a 3 bedroom place. For rental companies where there are only a few white goods supplied with the property then about £30 per site.
An earlier post said about £200 for a PIR and PAT, for an unfurnished let that sounds reasonable.
Are you obliged to fix any problems, no but you are obliged to disable any failed items and inform the responsible person. However as a standard offering replacing incorrect fuses, retightening terminals and other minor repairs i personaly carry out and retest included in the charge if done whilst on site. You are also obliged to provide a report showing the results of the tests and inspections a fault register showing faults found and any action you took .
Some of the faults i find are shocking (no pun intended) and i would say 80-90 % are found on the formal visual inspection.
Get a good PAT testing machine not a cheap fail or pass type and you can really start to add value to your customers. For example i found that one clients entire new office extension had had the wiring put in with incorrect polarity.
Finally as the first response said, go and get the qualification it protects you and your clients.