Main RCD???

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I would do as Ianmacd says , put the RCD on the shower , you have done your bit as safe as pos. You have made the customer aware , each set of regs are not retrospective, if they don't want it ,then as I keep saying, we are not the Electric Police.Its like a car mechanic I suppose, who tells you your car has four bald tyres and dodgey brakes. He can do no more, its now up to you, get the credit card out or ignore his professional advice .

Concur with Deke & ianApplaud Smiley

Divert the old shower cable out of the CU into a new standalone RCD...

run cable from CU MCB into this new RCD.

The circuit YOU are working on is compliant...

But you have NO obligation to have to amend any other circuits you have not worked on!!

I did a replacement shower like this a few months back...

just RCD the circuit..

advise the customer "in writing" and note on CERT.

no problem