I have worked in the windscreen trade for over 10years, any glass I get that's scratched we use jewellers rouge on a soft pad on a drill, is ok for a few marks but would take forever for a large amount of scratches. Always the chance of making it worse though, you will probably find its better or more cost effective to go down the route of replacing the whole screen. This would usually cost your glass excess if you are fully comp so should be between £50-£80. Also I take it the vehicle is a 2014 model? If you request genuine glass most insurance companies will honour it on vehicles less than 3 years old. Hope that helps