This was my second year of muscovies, The first year we fed them a mix of black oats, barley, (both bought locally) Alfalfa pellets, boiled potatos and pumpkin, worked out to be 16% protein, they also free ranged. They all lived and grew. Slaughtering was a disappointment however, at 20 weeks they were struggling to hit 1kg dressed weight for the boys. This year they got barley (locally bought) and organic Pig grower, which if you read the label is also for fattening ducks. They were slaughtered at 14 weeks and were 1.5 kg dressed for the boys. (I think I have pretty light muscovies dad isn't over 4kg standing. Again they were free-range so I would expect faster growth if they were caged. Ducklings should NOT get layers ration, it contains way too much calcium for them and can damage their kidneys.
I keep my hens and ducks together and they all get the same food, this means the hens are also on pig food, they have free choice grit and shell, small ducklings can't get the shells but after about 2 weeks I expect they can make it up there.
Grain direct from a farmer is the way to go. I can buy it from the feed merchant at 50p a kg or direct from the farmer for 10p. (I'm not in the UK so those are just to show the difference!) Get yourself some big bags, I use hessian potato sacks and check the free adds.