One can ask for a quote to install a new consumer unit, or a quote to make the property compliant. They are not the same. As to if RCD protection is required, one can debate this for years with no firm answer. However I have fitted RCD protection to all circuits, so I find it hard to say any one else should consider not to fit them, even if not strictly required.
The inspector has to make a professionally considered option as to if required. As @flameport
says in the main "RCDs are only used for fault protection on TT supplies." or other installations where the earth loop impedance is not low enough to ensure EEBADS (Earthed Equipotential Bonding and Automatic Disconnection of Supply) will work.
There is a big difference between not complying with requirements listed in the current BS 7671 for a new installation, and being potentially dangerous.
So if it passed last time, to become potentially dangerous then what has happened?
1) The last guy got it wrong.
2) Some thing has degraded over time.
3) Some thing has been added or changed.
As to what could be added or changed to require a RCD where it was not required 10 years ago, I have been scratching my head, however it is a brave inspector who decides he does not need to follow manufactures instructions. The rules have changed it now does not say you must follow manufacturers recommendations you only have to take them into account. However if the instructions for a Bosch boiler says it needs RCD protection one would need to be brave to say no it doesn't.
We did for many years have the drive by inspection and testing, it was money for old rope, it did not matter if a fault had C1, C2, C3, FI, N/V, N/A or LIM the owner did not need to act on any of them, however seeing the fines given when a fault has caused a death, the landlord would have been silly to have ignored points raised. What has changed is the English government has decided C1, C2, FI faults must be corrected or deemed as not required.
The law says the local authority if the work has not been done in 28 days must investigate and if it decides the work really must be done, tell you exactly what must be done, and you have 21 days to object, if you have not objected then 7 days latter the council can do the work and charge it to you, however there does not seem to be anything which details who pays for the council to do the inspection and list what must be done.
We then start to look at qualifications, if Mr. A says it must be done with a level 3 qualification and Mr B said it is not required with level 5, then the council would need to get Dr C to say it must be done. But Mr B is going to want a higher fee to Mr A and you need to be sure Mr B will not agree with Mr A.
So we get to the is it worth it? Do you want to pay £300 on the chance that Mr B will say it is not required?
So we are not talking about if right or wrong, it is more down to is it worth trying to get out of the work? Personally I would not rent a property at a lower safety standard to what I would want in my own house. If anything the reverse, as a foreman although I knew it was wrong, if there was some thing involving danger I would do it myself rather than give the job to some one in my charge.
I personally can't see why lights really need RCD protection, a tenant should not be drilling walls, or balancing on a full bath to change a bulb, so I fail to see why it is needed, although fitted a RCBO in my house, that cost around £6 more than a MCB so since changing whole consumer unit may as well for £6 extra fit RCD protection. Swapping a MCB for a RCBO if they will fit is likely to cost £30 plus labour.
There is no need for a new EICR all you need is paperwork to show faults corrected, and you only need to send the paperwork to the council on request, so simply attaching the receipts for the work to the EICR is all that is required.
411.3 I would assume the earth loop impedance was not low enough, so using a smaller MCB/RCBO would likely fix it, they are minor items to correct, it is likely many of the items are similar so simply by attaching.
3) Water leak corrected.
4) New MCB fitted receipt attached.
5) New screws fitted.
6) New lids fitted receipt attached.
7) Socket removed.
I am guessing what the real fault is, but it is unlikely the LABC will do anything unless tenant complains, and there is nothing to say who needs to do repairs so as long as not something which needs notifying then it would seem rather easy to get around the law if there is really nothing wrong, however of course if there is some thing wrong your up the creak without a paddle.