Pre-heat oven to 170C/340F/Gas 3 to 4. Grease and LINE with baking paper a large roasting tin or Pyrex roasting/lasagne dish - I use a 9 by 14 glass dish for mine.
Mix the flour, sugar, oats, ground ginger and chopped preserved ginger together in a large mixing bowl. Make a well in the centre.
Melt the butter in a saucepan and then add the golden syrup and treacle, warm them through but do not allow them to boil.
Pour the butter mixture from the saucepan into the well in the middle of the dry ingredients. Drop the bicarbonate of soda on top and then sprinkle the vinegar over the soda and watch it fizz.
Meanwhile, add the milk to the butter and treacle saucepan and heat it up gently - not TOO hot, hand hot is fine.
Add the milk to the ginger parkin mixture and mix thoroughly. It should be a fairly loose batter. Pour the cake batter into the prepared tin/tray and bake just above the middle of the pre-heated oven for 1 hour. (The parkin is cooked when it springs back when touched and is a dark brown with a sticky-ish surface.).
Allow it to cool completely in the tin/tray, then cut into slices or chunks and store in an airtight tin for 2/3 days before eating. It can be frozen at this stage too. You can eat it straight away, but it is always better after 2 to 3 days!