Many thanks to Colin Marshall who has given me tips and sent photos of his Graemsey. (A VERY professional build, I might add.)
One of Colin's tips was that he had made a spring pin for the ramp, which will allow it to be removed for painting or maintenance. This seemed an excellent idea but I couldn't fathom out how to do it!! Instead I have achieved the same result by making a bolt instead of a pin, which can be operated from the hatch. It seems to work OK so anyone building this model might want to try it.
Also Colin used magnetic strip to keep the hatch watertight. I have ordered some magnetic tape off ebay so will be trying that idea out.
This is the first time I have used styrene in a build and I purchased a face mask as so many people recommended. Unfortunately I wear glasses and the steam up almost immediately with the mask on. I am trying out a desk fan to see if that helps.