can you fill out the test sheet as unsatisfactory therefore failing the installation and then put you reasons in the extent and limitations box.
i would definatley finish the job then when testing it give him the cert with unsatisfactory on.
if he then wants an electrical installation certifcate he will have to get the work done.
i do think you have made a major **** up though by not having a look inside the house before quoting for the work. supposing he had an old rewireble bs3036 board would you have left that in as well.
i know people who have quoted over the phone before now i always have a good look at any job before i quote for it.
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