Like for like requirements

I think you have failed to realise the use of a MWC. Minor works are basically maintenance, changing something because it has failed, is damaged etc. Fitting a new class one fitting to an unsuitable circuit does not get an MWC because you can't do it, you will have to add the earthing conductor, or possibly an isolating transformer and that gets an EIC. The testing part would usually simply be functional unless a problem is seen, and in this case an EICR is the correct form, although you may well not examine the whole installation, particularly if it is a large factory or hospital.

The "like for like" idea is simply to point out that the MWC is really just to ensure continuity of documentation, so that one can record what has been done. Many electricians "forget" to do them for minor jobs, and certainly most domestic customers don't care. I assume you are asking because you are studying, the correct answer is my second sentence, reality is that one finds remarkably few with installation documentation. A large factory with a maintenance electrician should have several (10 maybe) a week, one for every mains voltage job he does, and exactly following BS7671 for every job with wiring whatever voltage that is part of the fixed wiring.

I think you have failed to realise the use of a MWC. Minor works are basically maintenance, changing something because it has failed, is damaged etc. Fitting a new class one fitting to an unsuitable circuit does not get an MWC because you can't do it, you will have to add the earthing conductor, or possibly an isolating transformer and that gets an EIC. The testing part would usually simply be functional unless a problem is seen, and in this case an EICR is the correct form, although you may well not examine the whole installation, particularly if it is a large factory or hospital.

The "like for like" idea is simply to point out that the MWC is really just to ensure continuity of documentation, so that one can record what has been done. Many electricians "forget" to do them for minor jobs, and certainly most domestic customers don't care. I assume you are asking because you are studying, the correct answer is my second sentence, reality is that one finds remarkably few with installation documentation. A large factory with a maintenance electrician should have several (10 maybe) a week, one for every mains voltage job he does, and exactly following BS7671 for every job with wiring whatever voltage that is part of the fixed wiring.

MEIWCs can be used for altering an existing circuit, for example adding a new socket in a bedroom. In that case all tests need to be performed on the MEIWC and the work needs to meet the latest BS7671. This may mean, for example, rectifying faults on the existing circuit or adding RCD protection and bonding.

I think it's been established that a 'like for like' replacement would not require the existing circuit to meet the latest BS7671 (commonly missing RCD, bonding, light CPC etc.), but should not make the installation any less safe and some professional common sense used.

What I'm now asking is if a MEIWC is used to document a 'like for like' replacement, a light switch for example, what tests would be required? And what happens if they fail? 

A failed test on an existing circuit where new work has been added would need rectifying for the new work to comply. But is this not the case for 'like for like'? If we accept that the circuit has no CPC or RCD but we are making it no worse, do we do the same for a failed IR test, or is the test not needed.

Testing IR, continuity, Ze and long leading bonding cables adds time and cost to the client, and if it uncovers failed tests then this adds more.