I make this regularly. It's enough for a square shallow tin that's 190 mm square at the top. And it is bloody good.
Melt in pan:
100 g butter
100 g Golden Syrup
43 g black treacle
43 g soft brown sugar
Mix together:
145 g plain flour
1 tsp baking powder
50 g medium oatmeal
7 g ginger (best to use 'drug-dealer's scales' 😃)
Beat together:
1 egg
2 tbsp Milk.
Mix liquids together, then add to flour mix, stir toll smooth and pour into tin.
Fan 140 deg for 45 minutes, turning at 25.
Handy tip: I put a long strip of baking parchment, the width of the bottom of the tin, in the tin before I pour. So on two opposite sides, it sticks out a bit. When you take it out of the oven, obviously let it cool first, but you can gently pull on the two loose ends to free the parkin from the tin.