The landlord regs has 2 main components:

1) the property must comply with BS 7671:2018 at all times (so installing an amd1-style EV charger isn't compliant!)

2) The property must be I&Ted by a competent person every 5 years, but this doesn't have to be to any standard, BS 7671 or otherwise. (A landlord could get an electrician to write a report assessing the electrical installation's compliance with the principles of Fung Shui and they would have legally discharged their duty.) However, if the report happens to flag anything as not being compliant with BS 7671:2018 then it needs fixing within 28 days.

Since very few dwellings in the UK are actually fully compliant with BS 7671:2018, the government then fudged it in the guidance, which said "of course older stuff is ok if it's safe". The landlord then has to rely on the goodwill of the LA or tribunal to follow the guidance rather than the letter of the law.

So in reality, an electrician should do an EICR to amd2, but reasonably flag things like lack of SPDs and AFDDs etc only as C3. Then the landlord should treat a C3 as covered by the government guidance which says its ok to partially ignore the law. By the landlord "choosing" to get the I&T done to BS 7671:2018 amd2, they're showing general good intent (as opposed to getting the Fung Shui guy in) and so they reduce the risk of the LA hassling them.

You I&T to the current BS 7671, so just now you (or your customer) have a choice. Whether what was compliant then, but no longer is now, is a C3 or C2 depends upon how out of date is the installation.

What the landlord does with the EICR is his business, not yours.

Yes but as I see it (others may disagree) the 18th Edition solely is mentioned in the Landlord Law then it prohibits the 19th when it comes in ounless parliament changes it or there is permission in place to allow the  Secretary  of state to vary it.

At the very least, you follow the test procedures in the current edition. So for example, you test RCDs at 1 x IΔn AC only.

I don't see any practical difficulties because items which were compliant, but no longer are would be coded C3.

I don't foresee any backward steps, but if for the sake of argument, the 19th Edn prohibited protective bonding, you might still award only C3. If C2, it would have to be disconnected, so I don't see a huge practical problem.

In any event, there will be a civil servant somewhere who will be keeping an eye on things.

The law explicitly says that the property must comply with the original edition of BS 7671:2018 at all times (it even quotes the ISBN number) - not any amendment. The law says nothing about what the I&T should be to: that is purely down to agreement between the landlord and the electrician - of course in the absence of any formal agreement, a landlord might reasonably assume the electrician is doing the "right thing" - and that usually implies inspecting against the current (amd2) edition of the regs.

Since it is a statutory instrument, it's supposed to be up to the relevant government department to keep it up to date, with the SoS signing it off and parliament giving a token nod. In practice, SIs are notoriously unmaintained - look at ESQCR, which still thinks properties are wired to BS 7671:2001 amd 1

Are there any differences regarding say SPDs or AFDDs in Amd 1 or 2 that were not in the original?

Since amd2, SPDs are now always required if risk of injury/life, safety service or financial/data loss.  They are required elsewhere unless the owner formally declares that they're prepared to risk it. AFDDs for <= 32A sockets required for hi-rise, HMOs, student accommodation, care homes; recommended otherwise.

So from a landlord's perspective, AFDDs are likely to become a thing if and when the Landlord Regs are ever updated to include amd2.

So if you follow the law and only EICR to 18th Edition no Ammds you do not consider SPDs?

No, the landlords gets whatever I&T they want. If (and only if) that I&T happens to find a non-conformity with BS 7671:2018  first edition, then the landlord is supposed to rectify it. So if the inspector writes a report which (in part) says "you have no SPDs or AFDDs!!", the landlord is under no obligation to do anything about it. Of course, in practical terms a landlord might not realise this unless it's clearly spelled out in the report, and the tenants might make a fuss.

This is common, there are lots of laws that  refer to previous versions of things.

It is not a mistake that the law references a particular version, out of date or not.  This is to ensure that the legally binding conditions are those set by the law makers when the law was written, not some unelected committee at the IET or BSI.

If the legislation had said 'latest version' then every time there is a new book of regs, the law changes, without parliamentary oversight Such 'ambulatory legislation' - you have to love legal English,  - is not normally accepted as it usurps the parliamentary process and would mean that the actions can be criminalised on the whim of a document review.

  Consider also  what might happen in principle if the regs writers were lobbied to change the rules to mandate some new and expensive ADS part only made by one or two vendors - Parliament may want the option to rule that in or out.

Test in the modern way, but read the results against what would have been permitted 2018.

Mike.