Hi Max, I guessed that the military bag would be (comparatively) heavy, so had discounted that.
Mike, take on board your Event comments, but I was looking at my budget, and Mark's suggestion.
Marl, the bag you saw was on the Field and Trek website (http://www.fieldandtrek.com/terra-discovery-lite-bivy-bag-783081?src=google)
It's still on sale at £80, and that's for a very light 3 layer gore-tex. Maybe not as good as the Event option, but a good price - Ronald Turnbull likes the full weight version, so hopefully this will do the business.
I'm not likely to be using it a great deal, so didn't want to lay out too much cash. The Rab Survival Zone was another one I had in mind, but time came for a decision, so the Terra Nova it is.