Rob, you are in luck! I have a Mk II and just done the windows. You can read about it here
http://www.tophatyachts.com/viewtopic.php?f=6&t=1766 I am not saying what I did is the best way but it is a way.
With the MkIII windows being bigger, I am not sure how much curve there is in the windows and whether that makes it harder to put on if you use the no-screws approach that I did.
Please note: the windows expand and contract quite a bit in the heat so they need a
THICK layer of the right sort of sealant to handle that change without breaking the seal. On my size windows the gap was 3 mm. Too many people make the mistake of just using ordinary plumber's silicon and screwing the window hard against the boat. This will leak. That was how my boat windows had been done in the past and leaking has always been a problem.
If you use screws, you still use some spacers to give you the right size gap.
troppo