If its just for a few sheep, then i doubt i would bother.
your livestock insurance will give you some public liability and cover from death by straying or lightning, theft etc.
how much are your sheep worth?
as for causing an accident, you arent resposible if the accident is caused by a straying sheep. as long as you have taken reasonable stpes to stop them escaping. the animal has its own mind and if it chooses to escape how can you be responsible for it. if on the other hand you have no fences or dont maintain them it might be a different matter.
if you have taken reasonable steps to secure your sheep and one was run over after escaping as i understand it you would be entitled to claim against the motorist! not the other way around.
we have insurance but my beasts have quite a value and cattle i think have a possibility of doing someone some damage.
dominic