I had a close look at the light unit today and, with the pointers from @craigoodwood and @MJI , can see how the cover should come off. However, I have decided to delay doing this until after the New Year as we need the car over the holiday period and I don't want to get into a position where the unit is unusable and I can't get the parts I need. Instead I have made an improvised back cover for the main beam light and polished the front of the cover to remove some of the clouding. If it stays dry, and everything still works after my fiddling, then I will refit it tomorrow and worry about it in a couple of weeks.

Thanks to everyone for their help so far. I hope you all have a great Christmas and a Happy New Year.

Just to wrap this one up. In the end I cleaned the light up as best I could and then replaced the missing light cover with a second hand one. You can see a bit of dirt on the inside of the lens but the light output is as good as the other light and at least it shouldn't get any worse now the cover is on.

Thanks again for everyone's help.