Oh how awful LadyK. I've had it run through my flock before, again brought in by a ram, so I now keep a closed flock, and quarantine any sheep brought in, for four full weeks.
Pink eye can come in with an infected animal so it sounds as if your tup brought it with him. To animals not previously exposed, it will affect the whole flock, but should give resistance for the future.
It is spread by things such as sheep feeding from a trough, where they rub against eachother, as well as by transference from the shepherds clothes or hands. Lots of other ways I'm sure.
My worst affected animal was also a Soay ewe. She was completely blind, with clouded eyes and scarring on the cornea. It took a long time, but she eventually recovered fully, with no long term scarring (and she was a total pain to catch).
We used antiBs (LA so injected every 3 days) plus Orbenin eye ointment as often as possible, at least twice a day (I see from the instructions on NOAH that you should use 1/4 tube per eye, once only, but may be repeated after a few days. I've never used quite that much) The Orbenin is good and by pulling forward the lower lid, you can get a good bit of it right into the eye, so it spreads well.
There is also a rather horrible technique where the AntiB is injected directly into the eyelid. Some people swear by this method, so maybe ask your vet.