spenceey-
I guess it's a vented or bonnet cap to prevent water ingress. If it was sealed there should be very little / no draft.
I looked in to this and asked around the consensus seemed to be that you need to be a bit careful if you restrict both the top and the bottom, especially if the top is completely sealed, or you are asking for condensation buildup, and on a long enough time line it could get through the brickwork and at the plaster.
The balloons are for semi-permanent installation and the wooly plugs are better for those who use the fireplace from time to time.
We have the vented caps and I've put balloons in the breast - but in summer I'll take them out so that the rooms and the chimney space can breathe again.
No mileage in putting anything else up there.
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Howard
What are you trying to achieve? The sheep or inflatable or newspaper or whatever will stop the draft. If there is one. Is your chimney capped at the top or ventilated caped or what?