Thanks to those of you who have changed the focus back from the fish to the cat! She has stopped eating jacks as well now: has fasted for five days.
As I said in my original note: she has had three blood tests, and thyroid is OK. As I forgot to say: liver / kidneys are OK too. That's why the vets (three of them) are baffled.
I am on the verge of having her put down: IF she eats, she may be fine, or may be in almost instant pain. If she does not eat (the last five days!), she can be her old, affectionate, confident self: but screaming for food.
If she eats for a few days at a time, she does put on weight. But then she stops again. If / when she starts eating after a long time without, there is a lot of vomiting at first (stomach shrunk & unused to food?)
I understand the point about taste. Can a cat have a thyroid problem without it showing up in the standard tests, I wonder? Does anything else 'remove' the taste of food?
Thanks for trying..