Greygate plastic polish was the stuff mandated by the RAF for aircraft transparencies when I last worked on live airframes about 20 years ago. I wondered if you could still get the stuff – now I know! I have used it for polishing glider canopies too.
Like Jon H I found that Brasso would be quite quick at removing stubborn stuff but don’t dismiss Greygate though. It can remove some quite deep scratches given time and elbow grease!
Regards, Aerial

Hmmm interesting…. *as he thinks about tackling the Sea Hawk canopy* 😀

Jon