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When did you intimate the complaint? What has happened in between? Have you been told why the satnav doesn't work?
This would appear to be a material breach of contract as it was essential to you that there was a working satnav for that vehicle and you made that clear. Unless there was two of you who witnessed the sale rep saying this, you would have a situation that it is your word against theirs but evidence in a civil court case proceeds on a balance of probabilities. You don't have to have corroborated evidence and it may be that the judge would find your evidence more credible than that of the sales person. Why would you have bought the vehicle if the satnav didn't work where the purpose of a motorhome is to go touring in places you don't know? I would sue for the cost of the installation of a system that does work and you may get an offer of settlement along the way.