Whilst I agree that the installer of the socket should be hung by the Gonads from a catenary wire until their bodily insulation resistance is a bazillion Gohms, there is nothing that you can do to change this.
Your EIC will not cover this s.o. as you did not install it.
On your EIC where it says "Extent Of The Installation Covered By This Certificate" enter "Please See Pg x".
Where Pg x is a page appended to the EIC detailing in a verbose manner the works that you have been contracted and paid to undertake.
On your EIC where it says "Comments On Existing Installation" enter "Please see Pg y"
Where Pg y is a page appended to the EIC detailing in a verbose manner the defects and limitations of the installation that you are not happy with and stating that you have advised the customer such.
Remember to number the pages on your EIC accordingly.
Also put on Pg y that the "offending" circuit has been left de-energised by leaving the mcb in the off position as you don't consider it safe or compliant to energise the circuit (or words to that effect).
You need to put that this has been explained to the customer as well.
Advise the customer in writing, with a letter to accompany your EIC to this effect, stating also that you cannot be held liable as you have advised them that this is unsafe, this will also be in your letter explaining that it is unsafe and why, include photocopies of any relevant sections of the Regs & OSG you feel appropriate, it is doubtful that you will be held liable under copyright in this instance.
Once you have done the job, been paid, and you are sending on the paperwork, scan, and photocopy (x2) the whole package, including the letters, keep the scan on your computer, as has already been suggested, send a copy of the paperwork to yourself in a sealed envelope and don't open it, ever.
Send the paperwork to the client by recorded delivery.
I feel this would be a robust defence and would allow you to do the work and cover your a r 5 3!
Thus making money, giving the client what they want, but, keeping yourself in the clear.
You could also when undertaking your training on the "new" board for your client explain how to turn circuit breakers back on if they are off
that way if they find a tripped breaker, they can re-energise it without bothering you.
That way you don't have to explain to them specifically to turn the breaker back on for the circuit you have left de-energised.
You can tell them that sometimes temporary faults occur, which they do, and these will trip an mcb, and when they are reset, then things are fine.
If they ring you tell them to turn it on and see what happens, just play dull, that you were not sure which breaker is off.
