Ah, thanks for that, Martin,
I went to a boat jumble in Littlehampton last weekend as I just happened to be visiting my daughters! And I managed to find a matching pair of round port lights for the princely sum of £65.
I had noticed that some Winkle brigs seem to have them on the front face of the cabin.
Am I right in assuming that this sealant would be good for fitting the new ones to the fibre glass?
I also bought a few other goodies as well but, that is another story.