From being the meat in the sandwich to being a secret ingredient

What is this obsession with the PDs and food? Eight months ago McDowell was telling us all that the PDs were the meat in the government sandwich, and now they've been relegated by the same man to being some secret ingredient.

It's not like you could hardly call the meat in your sandwich much of a secret. Unless it was a very small piece of meat, and a flavourless piece lacking in texture at that. Perhaps that is what he was getting at.