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!"
"Elementary, my dear Watson. From these footprints, I deduce that our quarry was a left-handed doctor of unusually short stature, who had known prosperity but had recently fallen upon hard times, and judging by the short, irregular length of his stride it is apparent that he had a mole at the counter."