RC took over a company that made a lot of special diet foods for dogs a while ago. They offer one that is designed for dogs with special cardiac needs -- this would fit many of our cavaliers once they have murmurs. Hence this may be of interest to some board members; the background on cardiac problems and diet is useful reading regardless. You can read more here:
This is a US site, but I know some vets in the UK/Ireland do carry these foods but you'd need to track them down. I would wager the RC varieties will be better quality than Hill's.
I see they recommend low sodium but NOT a salt free diet.