Cause of Death Unknown - can anyone advise?

I have 3 ex-batts which I rescued in aug 08, they have all recovered from their previous life in a cage and live in a run on the lawn. They have been laying very well (75 eggs last month) and all appear bright and healthy. This morning when I let them out of the Eglu one was lying dead on the floor. There are no visible signs of disease, wounding or injury. I'm not exactly sure of their age, but they are around 18 months to 2 years old. The remaining 2 girls seem fine, but I'm worried that I should be doing something to ensure they don't die suddenly too. Please can anyone advise why my hen died, and what precautions I should take with the other 2.

So sorry to hear about your sad news.

Poorly hens usually show some signs of being unwell (loose droppings, hunched up, lethargic and inactive etc). However sometimes they can just keel over suddenly. Have they had access to a new part of your garden. Could there be anything she may have eaten whilst foraging that is poisonous? 2 years old is not old for a hen so it wouldn't be old age which caused the death.

One of my ex-bats is very lethargic today, not eating, and very hot.
I have been putting worming stuff in the water the last two days, do you think this might have affected her, it's the herbal type?
I shall try fresh water with her, and see if she bucks up a bit.