Tim Downie wrote:
I normally try not to put any fat down our sink and when I do, I make sure I
use plenty of detergent & hot water

Better to leave fat to cool and then bin it I would have thought. Oil
and detergent is ok though.
Oil or fat plus caustic soda equals soap with a greatly increased volume
and, being only slowly soluble, it can create more of a blockage
initially.