Senior food is lower in calories than adult, and i have had success in getting cats to lose weight while on it - Felix do Senior food in normal size tins which is more economical when you are feedng multiple cats, although I have only seen this in a couple of supermarkets so far - I do believe Tesco was one of them. JWB is a good food, and also comes in a Senior/Light version. Exercise is important, and you could do things like hiding dry or using a treat ball to get them to get tehm doing more.