Yogurt, for those cats who will eat it, can work like a charm on digestive upsets. I used it on my irritable cat, until he decided he didn't like it any more. And I've used it many times when cats got diarrhoea from antibiotics. I found that I have to get the full fat for cats to enjoy it.

I can imagine how utterly frustrated you are at not being able to help Brodie. My favorite cat had IBS for several years and I had to switch diets on him about every 8 months as he became sensitive to it or stopped liking it.