This happened to us last spring: we had a ewe abort because of it. I know who the dog belongs to because of the description given by someone who saw it chasing the sheep, but there's no way I could prove it. It's VERY upsetting, but a few things spring to my mind:
1) It's worth having livestock insurance for this. It doesn't help with the emotional stress, but it does help with the vet bills and any financial losses.
2) Tell the police that the bystanders were videoing the attack. Do you know who they are? If not, can you give a description to the police?
3) Contact the owners. They should have calmed down and be feeling nervous and guilty by now. Tell them you don't want to sue them or to see there dog destroyed (which alerts them to these possibilities!) but you do need to know how they plan to stop the dog from chasing sheep again, and you do need financial recompense, even if it's in instalments.
4) Lots of talk about destroying dogs, crap owners etc. Realistically, just about any dog may chase sheep, and any dog may give its owner the slip occasionally. Accidents happen - but when they do, the owner should apologise immediately and mean it. And pay for it. And do their utmost to ensure it NEVER happens again.
End of speech, climbs off soapbox