I have at last managed to devote some time to tracking down a source for replacement digitiser screens. I will see if I can find a more local supplier but have ordered one up for myself. The screen is available
here.
I would advise anyone considering ordering one to drop the front off the Nav' and verify the part number is the same. It is clearly visable once the sticky tape is peeled back a little. I have posted an image for anyone unsure.