hi plumseverywhere,
one of the main reasons that people say sheep "drop dead" for no reason is that a range of the diseases they can catch can present mainly as a sudden onset collapse and quick spread of infection and/or sudden death wih no obvious signs beforehand. A contributing factor is that in some cases these diseases can cause several sudden deaths or collapses in a group within a short space of time, which is tragic and very dramatic to behold.
Classic examples include clostridial (bacterial diseases) in unvaccinated sheep (the bacteria produces a toxin/poison that kills the sheep very shortly after infection) - these are found in the soil.
"Pasteurellosis", a group of bacteria that cause a fast spreading septicaemia ("blood poisoning") or pneumonia in sheep, especially seen in spates of sudden deaths after moving or bad weather
Fluke - Cows tend to show signs of having this parasite, whereas I have seen countless sheep, some in great body condition, that have been put on contaminated pasture and eaten loads of the baby parasites and ended up with a totally trashed liver and subsequent death within an alarmingly short space of time.
And- in the case of a single or a couple of deaths within a short space of time after moving to lovely lush pasture such as clover - sometimes the diet change causes gas to build up in the stomach and rotate the entire digestive system, cutting off its blood supply and causing death.
There are other causes with similar presentation too!
The innards of a sheep= complicated things! Hope this helps. There are post-mortem pathology labs around the country which are subsidised by the government and can do top-quality post-mortem investigations to try to find the cause of death for a very reasonable cost similar to a veterinary visit- most vets, when asked, can arrange for this if appropriate as the labs work alongside the vets to help and provide advice, and not in competition with them.
Examples include the government subsidised SAC in scotland, and the AHVLA in england.