Go raw – we use Natural Instinct – the main cat eats Venison and the kitten is on organic chicken.
Our old cat was on special diets but still had terrible digestion and number 2s, in his final months we just feed him raw chicken (not specific cat food) and his digestion and toilet improved massively to the point of no problem, and he also bulked back up a bit.
Even that waitrose stuff is only 14% meat whereas the chicken we feed the kitten is 98% human grade meat. Even Serena Williams could eat it.
And the hipster stuff is probably not much better.
It’s also not that expensive as a 500g tub lasts 4 or 5 days and costs about £2.50. You just can’t leave it down, so portion so they eat it all, and then give them seconds if required.
http://www.naturalinstinct.com/raw-cat-food
The Pure foods are not complete foods so you need to mix them up a bit, the complete ones have taurine and other stuff cats need.