mapj1:
This depends on how the flat is used - if it is to be rented out then since earlier this year there is a need to meet the current wiring regs before it can be let out ...
I was assuming that this is what it is about.
I don't quite agree with Mike - what is needed is a "satisfactory" EICR.
Concerning the cost, obviously it depends upon the size of the flat and the number of circuits, but given that the EICR has been done, that is a good half of the work (assuming that everything else is in order).
On the basis of the age of the CU, it is unlikely that it will be possible to replace just the main switch, but it may be possible.
Out of interest which domestic sockets do not need RCD protection.
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