Well I've managed to establish that there were in fact two couples called William and Sally/Sarah Cock - the ones in the Mevagissey area had 3 children, one of whom was a John, but apart from finding her maiden name (Billing), I don't know if or how they were connected, as yet. That can wait.
I've added the census info re Frances St. (- so it looks as if what is now the Wig & Pen was once a grocer, although according to the pubs of Truro article, it was also called the Star Hotel in 1864, so perhaps became a grocery store for some time before turning back into a pub). Being on the corner of Frances and Castle, I wonder if in fact the different addresses recorded for various Cocks actually refer to the same Cocks? I will have to go through those again...
I've hit the wall with the father of the William Cock who was married to Sally Kellow, as far as parentage is concerned. There are no likely baptisms in Truro; one in St Just in Roseland (1760) and one closer to Sally's neck of the woods, at St Minver (1752), but there are a lot of other potential candidates and I can't think of a way of linking any of them definitively, or at least more strongly, with my William. Will have another look with fresh eyes tomorrow.
Edit: A quick last note. Looking at the history of the Wig and Pen in the pubs article, it was a pub called the Star in 1864 and then again in 1933. However the Census records of 1851 and 1861 seem to indicate that there was a Cocks' grocer shop in Frances St at that time, and in 1881 Edward was a grocer at no.1. Perhaps there was more than one dwelling involved?