Ok - to all of those who say - "don't use fertiliser" - you are quite right, the grass will grow without any. But if you want a bit more grass, or to give it a boost in spring, then adding a small amount in spring (ie now) is the answer.
The original post asked how much and when - not who puts fertiliser on and who doesn't
So even though there are only 13 ewes on 5 acres, I bet there wasn't any spare grass after last year's dry summer, and who knows, perhaps some of the lambs are to be kept.
For those of you that suggested getting some manure from a neighbour - you don't plaster your grazing land with someone else's manure, thus making it inedible to your own stock until it's washed off or new growth has appeared (Which again was in short supply last summer) Apart from the fact that the OP has said they have small fields, with difficult tractor access, and will be spreading anything by hand, you surely aren't envisaging them spreading muck with a wheelbarrow and fork?
Even grazing land needs nutrients replacing, and the main one that grazing animals take up is phosphate, which also, as mentioned will encourage clover. Potash is largely put back in the soil on grazing land, in the form of urine, and is generally needed more where you are taking a grass crop off as the grass (and potash) are then removed from the field and need replacing. This is where ruminant manure would be useful as it is high in potash.
So - to answer the question - I would put on about one bag/acre of either 20-10-10, 25-5-5, - which are general purpose blends, or - specifically as a grazing one which will encourage clover - something higher in phosphate like a 20 -15-5 . You can apply it now, as the grass has started growing and will take it up. Also, it is not poisonous to grazing animals so you do not have to remove them from the land where you have applied it.
The figures mentioned refer to the amounts of nitrogen, phosphate and potash, in that order.