If it goes into a barn, get some people together and trap it in there. Then read it's ear tag numbers and phone your local Animal Health (should be part of Trading Standards at your local council) and give them the numbers. Also tell them that it appears unwell. They will be able to find out who it belongs to, and they should call them and tell them to come and collect it asap. (do this on a weekday not a weekend!)
No point you wasting your own money on vets etc. Yes, the sheep will be stressed out by this for a short period, but if it means that it can be caught and returned to its owner and made better then its the best option.
If you can't muster up any bodies to help you trap it, you could call the (dreaded) RSPCA to help ... they do like a good run around after a sheep

.
Other alternatives would be chase the sheep down the road to the farm that you think it's from and leave it there, but that doesn't guarantee a good outcome for the sheep, and it would be a bit rude if it turned out not to be their sheep.