Formula change!!! That explains it!!
Seeing my cat produce yet another heap of diarrhoea in the litter tray today, I had been wondering why the food didn't seem to be working as well, and whether I should try another brand. Well, now that I have just read the reviews about a formula change, I realise the reason. Such a shame, as this food had quite a miraculously beneficial effect when I put the cat on it a couple of years ago. No other food was ad effective, so I now feel a bit desperate.