Hi Nigel,
I fitted a different type of cable gland in a similar position on the port side of the sugar scoop on my B36, and while I didn't measure the thickness of the GRP, I'd say it was getting on for about 10mm as the self tapping screw ends provided with the gland did not reach all the way through. As yours is a bigger boat I think the thickness of the GRP would be commensurately thicker.