It really rather depends if there was a good reason for the 3 years interval  - is the building in an unusually  poor state, (rusting conduit flaky rubber cables ??? ) are the occupants unusually likely to damage things or especially vulnerable?

If not then it is reasonable to ignore the report and stick with the industry standard guidance of 5 years, performing some intermediate visual checks in between as an act of due diligence. You have to ask what sort of thing is likely to go wrong - a pure visual check for smashed light fittings and burnt sockets etc and perhaps a flick of the RCD test buttons, should be occurring as part of tenants moving out and in anyway in a well organized set-up, and that does not really need an electrician, especially one who as you say makes glaring errors.
Even if something happens within the report period you have very little comeback on the inspector anyway, unless it was actually wrong at the time of inspection and they missed it - and that could be realised at any time afterwards - so for example a missing RCD or a live light fitting they should have seen, one they tested that has since failed in service, well that could happen at any point. The risk of the latter is incredibly small - until 2 years ago there were sites that went many decades between inspections, and there were no great excess of accidents. What is important is that if something is reported either by the tenant as defective or on a walk round by the rental agent, that every one knows they must call it in, and to whom.
Mike.

Thanks for the response. 

Sadly there was no rhyme or reason for the reports being 3 yearly. There are a mixture of satisfactory and unsatisfactory reports, so it's not even like the more dangerous ones have been recorded as shorter. The guy just either didn't know what he was doing, or was attempting to generate more work for himself. 

My point really was whether the client could be held accountable legally for the previous tester's error. Being rented accommodation the law is quite clear on this; electrical systems must be maintained at all times - the only way of proving this is to have it tested when required and to keep a record of it. So, as unlikely as it may be for an incident to occur, if it did then our client would have to demonstrate they have taken all reasonable steps to ensure safety - or face potential prosecution. There's also every chance for insurers to refuse any payout. 

So essentially, are they satisfying their legal requirements by sticking to the recommended test intervals by IET, NICEIC etc (5 yeras), or does the albeit erroneous test interval plucked out of the sky by the previous tester take precedence if the worst was to happen? 

Sadly there probably isn't a black and white answer.

Thanks for the response. 

Sadly there was no rhyme or reason for the reports being 3 yearly. There are a mixture of satisfactory and unsatisfactory reports, so it's not even like the more dangerous ones have been recorded as shorter. The guy just either didn't know what he was doing, or was attempting to generate more work for himself. 

My point really was whether the client could be held accountable legally for the previous tester's error. Being rented accommodation the law is quite clear on this; electrical systems must be maintained at all times - the only way of proving this is to have it tested when required and to keep a record of it. So, as unlikely as it may be for an incident to occur, if it did then our client would have to demonstrate they have taken all reasonable steps to ensure safety - or face potential prosecution. There's also every chance for insurers to refuse any payout. 

So essentially, are they satisfying their legal requirements by sticking to the recommended test intervals by IET, NICEIC etc (5 yeras), or does the albeit erroneous test interval plucked out of the sky by the previous tester take precedence if the worst was to happen? 

Sadly there probably isn't a black and white answer.

I agree it  is unclear, but the local authority, as the potentially prosecuting party, may be able to offer some reassurance. Certainly round here they are well aware of problems (not like this, but certainly with a shortage of good inspectors and of EICRs of dodgy technical merit,) and have no intention to prosecute those landlords making sensible and proportionate efforts to maintain their buildings. *

Mean while I'd be inclined to write or call the inspector back and say 'why 3?' and see if he or she can make up a good reason. It is not a question they should be fumbling about, and having presumably paid for the inspection, then it might be wise to testing the technical support behind it.

Mike.

* I have a small flat I am in the throes of trying to rent out, and am now seeing all this from the landord side, there are quite different set of driving factors.Part of that has involved quite some conversation with the local authority.