As in Mrs. Fitzherbert (1947) and Duel and Duality (1987), King George III is played by a German actor, although he was the first Hanoverian King of Britain to be born in England and speak fluent English (unlike George I and George II).
St Margarets Bay is on the other side of Dover to the Romney Marsh (Dymchurch), instead of "this side of Dover". Although a bit close to Dover itself, it could have been Shakespeare Beach - (Duncan an ex Dover Shartk)
Ransley and Dr Syn are seen drinking out of aluminium cups in Ransley's house. In the 18th century aluminium was more precious and rarer than gold.
When Syn and his party enter the prison cell, they close the door behind them (where presumably it remains unlocked); yet, the prison guards do not notice the door was unlocked when they check in on the cell inmates. When the guards leave, they apparently also leave it unlocked, since Syn and his party have no trouble exiting.
The way into the "secret stable" in the abandoned farm is via some floorboards: it is difficult to explain how the horses themselves could get into this underground chamber.
When Syn and his party enter the prison cell, it's night. A torch already burns inside the cell. That's damn considerate of the guards to waste fuel and leave the prisoners a night light when they are otherwise treated very shabbily.