I really don't think it's diet related in this case, and I think your vet should be looking at other causes....of which their are many, but diet is the one that vet's always grab at first
Known causes include obesity or hyperlipidemia, trauma, decreased blood flow to the pancreas, toxins, medications, chronic kidney disease, infectious agents, pancreatic duct obstruction, and neoplasia.
It's more common in middle aged bitches, and cockers are a breed that are very susceptible to it.
Personally, although I don't rate Burns for my dog (he didn't thrive on it) it is a good food with a very controlled fat content and one that I would rate as better in fat content than Chappie. I would contact Dr Burns via their website and ask for their advice too as I'm sure he would be interested to hear that his food is being connected to a Pancreatitis attack
and Fiona, I would think that Gari's tumurs bought on the pancreatitis