my short answer is Nauticat themselves. I have trawled the web and phoned and visited numerous chandlers/hardware stores etc etc etc. However storm hinges of the small size you probably require seem to be nowhere in the UK. So I contacted Nauticat with the exact dimensions and bought four. A word of caution, they are eye wateringly expensive and so is the postage. I can let you know how much if you want, but I would not like to spoil the surprise.
I needed locker hinges for my Nc39 back in 2012 and conducted a similar wide search as Sam did. Eventually I bit the bullet and got the spare hinges from Nauticat themselves. Expensive, but needs must!
However, later on I did find that a company called Schwepper, http://www.schwepper.com who sell similar ones to those on my Nc39 locker doors. Look in Chaper/Section 5 of their on-line catalogue. I have no idea of the cost or availability. They have a wide selection.
Are replacement hinges essential? I had a locker door in my 331 which was not fitting properly. I found that the hinges had been distorted but managed to square them off again using a small vice, hammer, drift and grips! The door closes fine now.