Sorry for not giving much information - I never expected such detailed and considered replies. I'm very grateful for them.
The GRP/petrol combination, is what I can afford so I must accept its limitations. Family circumstances mean that my boating will be within these limitations, anyway, I think. I'm still in the very early stages of looking into it all and know nothing, but I was thinking of a marina when not cruising, with shoreline availability and recreational boating to begin with, only a night or two at a time. But I will look out for diesel boats too, and see how they compare price-wise.