The contents of a well-used but badly flushed lavatory, the bangers (sausages) being the turds and the mash (mashed potato) being the bog roll.
"Cinders and ashes, Thomas, your coaches are a disgrace", cried the Fat Controller. "There's soot all over the windows, used tickets all over the floor and Clarabel's khazi is full of bangers and mash!"
Made from the word "banging", bangers and mash is simply one step further. To describe something that is better then good we use excellent, to describe someone more cool then cool we use ice cold and to describe someone or something more banging then banging we use bangers and mash.