So that's another thing to add to my little list of the ways vets con money out of us then. Thank you David, that's useful to know.
On the exclusion principle, and having tried six different highly rated foods, it looks as though the Little Cav probably has an allergy or intolerance to peas and maize, given that one or the other was an ingredient in all the foods he had before Nutriment and all gave him problems while Nutriment contains neither. I've been through all the ingredients of each of the foods again and nothing else fits that bill.
I suppose I would have to try him on each again to be certain, but frankly I'd rather just avoid them.