Marmite is good for sweet-itch sufferers. My large cob hardly suffered at all last summer since having a couple of teaspoons a day in her food (and wearing a good fly sheet).
I get the kettle and boil it. get a teaspoon of marmite and twirl it round until there is none hanging off it- much the same as you would with golden syrup! Stick the teaspoon into a mug. Fill to the top with boiling water/ Watch it melt. Let it cool a little. Add to handfull of chaff and bob's your uncle!
I fed Marmite last year - I got fed up with getting all gooey and bought the squeezy jar, and put a squirt in his feed. But this year I have bought a tub of brewer's yeast, which will be cheaper and I understand has the same effect. He likes the taste. He liked the Marmite, too.