The was a major change where bonding was relaxed if there was RCD protection, we see a lot of homes where the bonding has been lost but the RCD not added, this means a home which seems to be as it was in 2008 often is not as some plastic pipes has resulted in bonding being lost. So although BS7671 with every edition gives a design date after which any new installation needs to comply with it, so not retrospective, and a C2 code is potentially dangerous, not does not comply with any regulation, so if it complied in 1992 then it still complies today, so no requirement for RCD protection unless bonding has been removed. However I fitted RCD protection to my own house all circuits in around 1994, and had mothers house rewired in around 2016 so I could fit RCD protection, and this house RCD protection added within first 6 months of moving in. If you look at the Emma Shaw case there was a whole host of faults which caused her death, and at the time of her death RCD protection was not required on all circuits, and it was the electrical foreman who was blamed for using unskilled labour, however had RCD protection been used it would have stopped her death. The tenant does not really have the option of fitting RCD protection only the landlord can do that, I think a 28 day limit to get it done is unreasonable, and the landlord should be free to arrange when it is done to suit other things, including allowing tenants to run down any freezer stock, where to upgrade it requires a CU change, but even if not legally obliged, morally however they should be fitted.