Sure enough, C Ale gets a mention. This is the paragraph describing Groves & Whitnall's bottled beers:
"Burton itself cannot produce anything which, in its class, is superior to our Bitter Beer. Our famous "C" Ale is similar to the best Burton "Mild". Our strong Old Ale or "Barley Wine" stands pre-eminent and apart from all others, and our "Special" Ale in bottle is fittingly described by its title."
So C Ale was like a Burton Mild. That's progress, of a kind. I wonder what exactly they mean by a Burton Mild?