In reply to BusyLizzie:
Intriguing. I also thought Ransome, but no, and I have scoured the back covers of my Puffin Posts from the early to mid 70s, on which all the new titles were listed, but nothing fits the bill. BB, also on my shelves, pretty much only did animals and gnomes, I think. Shenanigans of the sort you describe on Scottish islands sounds a bit Alastair Maclean, or even Tintin, but the Venn diagrams don't quite line up.