I think many on this Forum will be amazed at that Monte Carlo! Very nice. The crystal is acrylic, if I am not mistaken, and the scratches can probably be buffed out using some Polywatch polish. That said, the crystal is considered consumable and can be (should be most likely) replaced at service with a period correct version. There is either a lot of DNA accumulating under the edge of the crystal or a little rust. I'm looking at the edge of the crystal from 2:00-6:00. I don't know the vintage specialists in the UK but in the US there a are a few; the Ridleys, Ric Deital, LA Watchworks, etc. I'm sure you will get many references for service momentarily. BTW, this should be listed in the vintage section for the experts. I believe there is a sticky in the Vintage Forum for repair specialists around the world.
