The heart has always been associated with emotions, especially love, excitement and fear, probably because when we feel a strong emotion we feel our heart thumping and beating faster.
So when something "warms the cockles of our heart", it is a reference to something pleasant that makes our heart beat faster and makes us feel good.
The only difference between a mist and a fog is the visibility.
The cockles of the heart are its ventricles, named by some in Latin as "cochleae cordis", from "cochlea" (snail), alluding to their shape.
There are left and right auricles (now referred to as atrial appendages) which are continuous with the left atrium and right atrium (the upper chambers of the heart) respectively.
They are believe to be remnants of the left and right atrium of the developing heart when we were embryos.
The group won Eurovision in 1974 with Waterloo, which became their first number one single in the UK.
One of the meanings of "cockle" is the chamber of a kiln: in reference to the heart, it refers to the chambers, of which there are four (two atria and two ventricles).