What exactly does that saying mean, I have never really known.
The Lord Mayor of London shouldn't be confused with Ken Livingston's job, its a ceremomnial position to do with the commercial City....and as such when a new one gets in place, they have a big poarade of old style coaches, drawn by horses.
Ther horses dump a load of shit everywhere as they pass....so its the shit after the Lord mayor's show. Pointing out the contrast between the fine costumes and other decorations.....and a load of shit left after its over.