Good read but a few inaccuracies and plot holes. The villain travels by canal from Norwich to the Fens along canals with locks. There are no canals in this area, just rivers.
The island is obviously fictitious, which is fine, but off the coast of Aberdeen. He mentions one of the heroes picking up a boulder from chalky soil on the island, but their is no chgalk in that part of the UK.
The spycatchers track him to Aberdeen where they realise he may have escaped by boat, but none of the spycatchers thinks he might be on the 10 mile long island which is only 10 or 15 miles offshore with high cliffs. Surely they might have guessed that he may have gone there?