Hayley,
I did ask you to look at the basis of Contract as "missing" something having given a quotation means the loss is potentially theirs NOT yours, hence my further question about E&OE.
If they have given you a firm quotation based on a survey then Contractually you have every right to challenge the need to pay for extra work that legitimately could/should have been identified at the time of quotation.
That is the difference between a Quotation - which is a firm price and is binding, and an Estimate which is an indication but is not binding. So in your case if they have said no need to sort out any electrics, then should they subsequently find that this is not the case you can refuse payment and they have to complete the works at the quoted price because they were given the opportunity to inspect and a competent contractor would have carried out their due diligence properly. If the have failed to do that then the loss is upon them NOT you.
This is different to a case where something unexpected had been discovered that would not have been obvious to a competent person. For example, an electrician is asked to fit metal light fittings to a 1960/70s house. From that era it is likely that there is no CPC (earth wire) in the cable. If during the work he finds that to be the case, the remedial work is entirely down to him because he should have known or made sufficient investigations prior to making a quotation. There is substantial case law on this precedent. Hence why I asked you so many questions.