Ad lib hay is great, licky bucket great (esp if its Crystalix), browsing the hedge is great but I would say that that much coarse mix is a bit excessive for Soay. The only way to tell is to check their condition score, which in primitives should never be more than about 3, but more like 2 1/2. Don't suddenly change the amount they are getting, but next year you could monitor how much you give more carefully, by seeing what their condition is each week on whatever rate of coarse mix you are giving them and adjust accordingly. They don't need any coarse mix for most of the year - the routine is to introduce it gradually at 6 weeks prior to lambing, slowly increasing it up to lambing time. This partly compensates for the space taken up by the lambs which prevents the ewe from eating enough hay, so to get adequate nutrition we give it in a concentrated form. The exception to that is if there is severe weather with thick snow lying when a little coarse mix is needed for the whole flock.
I don't want to give actual quantities per sheep as so much depends on individual circumstances, but definitely far less than a big commercial ewe would need.