I've just started to feed mine hay. Quantity depends on how much grass is left and if they are lambs you are feeding up for slaughter or older pets where you are looking to maintain condition. If I have a hay rack available I just fill it and wait until it is empty as they don't tend to eat it unless the grass is short. If there is no feeder and it is on the ground then a large barrow load a couple of times a day to start and increase if it is getting eaten too quickly.
Sheep nuts is kind of optional with Ryelands but a handful a day to,start and increase when the weather turns cold