Have Halfords ripped us off and or Broke the law ?

Don't use the radio until it's sorted. Keep it turned off. Get your money back from Halfords by quoting the Sale and Supply of Goods Act and your reason for wanting a refund is that the service was not fit for purpose. Make sure you have your receipt and any other relevant paperwork. If Halfords get difficult, take the car to a reputable ICE (in car entertainment) dealer and get them to sort it or at least get a quote, then go through the small claims court to recoup the money from Halfords. Often mentioning the words "Small Claims Court" can wake up a retailer so be sure to advise Halfords of your intentions if they don't come up with the refund or a satisfactory repair. In all probability the "unknown reason" they couldn't sort it was that the only Halfords engineer who knows what they are doing in that branch was off that day.