ambuka - Sheep are born to die i'm affraid, get yourself in contact with a vet, go to your local one and have a chat with them, there are several vets in a practice so if you dont get the answer you want from one try one of the others
Alot of farmers these days are Organic so the vets should be able to advise you on the best treatment. Ask your vet what you should be vaccinating against, dosing with, maybe even ask them to make you a program for the year of what you should be doing with your sheep.
You can do worm egg counts so that you dont have to over worm and get resistance to a particular drug, Have you heard of Coccidiosis?(like a worm) not common in adult sheep (as they grow resistance to it) but in lambs it can be a killer if not caught early, Fluke also, most common on wet pasture but not always.
Vaccinating against pneumonia and chlostridial disease should be a priority in my opinion, do you know the history of the field you are grazing? were there lots of sheep on it before you took it on? Disease can live in pasture for years. You can also vaccinate against foot rot. Dont forget Blue tongue vaccination's too.
With regards to pneumonia, if bad enough it might never completely go away without really strong antibiotics (stronger than terramycin which would be usual treatment)
If you get another loss, which I hope you don't, get your vet to do a post mortem so that you can find out why it died.
I hope I have been of some help to you, I have kept sheep since I was a kid and every year I get losses in some form or another - it's part of farming - Live stock - dead stock
you never get used to it but it's part of life.
Like others have said, get a good book, get friends with your vet and a neighbouring farmer or two!!!!! and dont be affraid to ask lots of questions
If you need a chat feel free to contact me
Good luck with your sheep
Ffion