What qualification is required to inspect against Edition 18?

AFAIK, there is no statutory requirement to hold any particular qualification in order to write an EICR. However, a wise customer might want to be assured that the  inspector (and tester) has the appropriate C&G qualification (which change from time to time), or is a member of a registered competent persons scheme, or a combination of the two.

The C&G in I&T might be regarded as long-term because the principles do not change. Knowledge of the current Edition of BS 7671 is demonstrated by C&G 2382.

Chris Pearson said:

Knowledge of the current Edition of BS 7671 is demonstrated by C&G 2382

There's some disagreement with that statement ... although if someone has "underpinning knowledge" (prior learning) to demonstrate the understanding) then yes, adding the qualification probably demonstrates a general understanding of the latest BS 7671 ... but this then raises questions as to how long ago the "prior learning" was taken, and whether it covered some concepts in use in the latest BS 7671 (e.g. Solar PV, EV charging, Prosumer's Electrical Installations etc.) ... 

... which is most likely why "bolt-on" qualifications are becoming popular.

So, does it depend on whether the installation has any of these "bolt-on" components as to whether the individual is competent to undertake EICR on the whole installation ... or are we going to see increasingly piecemeal EICRs ("this not included, that not included")?

Hm! At least on the day of the exam.

Given its open-book format, I suppose that you could argue that it does not demonstrate knowledge, but at least the candidate knows where to look for e.g. current-carrying capacity of cables.

There may well be electricians who would decline to inspect (and test) an installation with PV and EV and still make a living. The folk who have been doing my MOT tests for nearer 30 than 20 years have not got involved in EVs, but you still have to book well in advance.

Hm! At least on the day of the exam.

Given its open-book format, I suppose that you could argue that it does not demonstrate knowledge, but at least the candidate knows where to look for e.g. current-carrying capacity of cables.

There may well be electricians who would decline to inspect (and test) an installation with PV and EV and still make a living. The folk who have been doing my MOT tests for nearer 30 than 20 years have not got involved in EVs, but you still have to book well in advance.