When Sir Thomas Langley writes to Queen Isabella, he folds his letter many times and seals it with wax. When Queen Isabella receives the letter it is on rolled parchment and tied with a string.
The "poisonous" mushroom used by Petranilla to poison Caris' mother is clearly a Chanterelle and very edible. The only lookalike to a Chanterelle that is mildly poisonous is a Jack-o-lantern which only would give cramps but would not be lethal so a poor choice if you would want to kill someone. Jack-o-lantern is also not indigenous to the UK and almost impossible to find.
King Edward III, the man being crowned King at the beginning was actually only 14 years old when he became King. The actor who played him was 32.
In the chase scene near the beginning when Langley is being chased by two men on horseback, the men are reloading Crossbows while riding. A crossbow has a much higher poundage than would allow a user to reload while riding, that is why the little triangle of metal is at the bottom, it is for bracing the crossbow with your foot to redraw it.