If you have been feeding fresh fish up until the onset, is it possible that the fish could have been contaminated? In the states, there is a risk that fish could have been infected with mercury or toxin from pesticide run-off that enters the fresh water supply, as well as bacteria such as e-coli, among other things.
This is just a thought, as I don't know how the fish in the UK compare with fish in the US. I am also not terribly educated on the fact, as I never have bought fresh fish...I don't care for it and neither do my family members.
Did the vet check for any possible contaminants/toxins that may have been ingested - either via the fish or perhaps some other means?