A winter-set movies in the UK when deciduous trees are shown in full leaf. The exterior shots regularly feature this error when everyone is used to bare-leaved trees from mid November through winter.
Around 1h 5 mins in, a man wearing a medical face mask is visible in the top left corner.
It is Christmas and when Peter Hope exits the front of the house the trees are in full leaf.
In the Plough Inn bar scene, a crew member is visible with his mask on at the top left side of the screen at timestamp 40:23.