What are Hull Hotcakes?
I've found a couple of plausible candidates.
The first (and I think more plausible), prompted by a twitter thread is an oven-bottom bread-cake*.  Here's (Facebook, with discussion buried in reminiscences, but no recipe I could see) an old photo showing them at Drewery's Bakery.  Similar things exist across northern England.  Until recently they could apparently be bought from Dixon's Bakery in Hessle Road.  It's worth looking at other traditional bakers, though the most likely I've found are currently closed.
The second is related to the clapcakes or havercakes found in other Northern counties.  These are made largely with oats, like the Staffordshire oatcake.  They sound like they're rather flat (even pancake-thin, and may use a little baking soda (recipe) or yeast (various sources but no recipe online that I've found).

* For anyone confused Bread-cake is a Northern English term for some types of bread roll.