I had the same problem on my +8, the previous owner had kept the wipers going with a missing blade. I could see the scratch and feel the scratch with my finger nail. I used the kit from Frost, basically a polishing wheel to use in an electric drill and some cerium oxide powder. This worked well, although it can be a bit messy. Unfortunately Frost are out of stock of the kits at the moment. Faced with a similar problem on another car I bought a kit through ebay. The scratches were a bit deeper and I had to use an abrasive pad before I finished with the cerium oxide. Took a while, but again it worked OK. The kits do mention that you need to use a mains powered drill, as they suggest the battery powered drills do not have the necessary power, or longevity. On ebay just search for 'deep scratch remover, glass polishing kit'. Try the powder first and then use an abrasive disk if the powder isn't man enough. You will then need to finish with the powder.