I very much doubt there's anything legal you can do with the Sale of Goods Act what with the nature of the problem (I.e not a defect that the garage could have known anything about) so I very much doubt returning the car will be an option.

If however the warranty small print includes electrical items then they have to rectify it. They need to send it to an auto electrician who can fix it. If it does it again video it, showing that the interior hazard switch does nothing, as I presume that they haven't replicated the fault, so may not believe the problem exists.