The first thing to do is run your fingernail over the scratch. If your nail gets caught, it's probably too deep for any kind of home remedy or repair kit. If it doesn't, try one of the many windscreen scratch repair kits available online or in stores such as Halfords. For the best results, though, an at-home repair company such as Autoglass would likely do a better job. The windscreen is safety-critical, and a scratch that impairs the driver's view could be an MoT failure.