From what the OP has told us, I suspect that it's too late for that. In any event, if he made an offer in the knowledge that "the sockets and switches look suspiciously old apart from in the extended area of the house which is slightly newer. ... All the switches and plugs are the yellowy plastic type, totty switches, everything just looks old." and that a re-wire might well be necessary, I don't really think that he would even have a very strong moral argument for reducing his offer, even if he could.
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