Thanks Tsu.
I think I have come across some fishmonger Collis cousins, but can't remember who they were, and unfortunately I've not found an easy way of finding people by occupation on the Family Historian software I use.
Another branch were greengrocers and I had a slightly bizarre experience the other day: I was driving through the vale of Evesham and thought to myself "oo, it's asparagus season, must look for a farm or roadside stall selling some".
I screeched to a halt outside the first roadside shed I saw, bought some (delicious!) asparagus, but it was only as I drove away that I looked back at the sign over the shed: R Collis & Son!!!