Jo wrote:

Does anyone have any experience of chimney balloons


Yes.

as we have heard that these are quite effective?

Are you not sure that you heard it?

Anyway whoever told you that was a downright liar. They are a pointless
waste of time and money. If you get a good strong wind as we have today
you will find the balloon being sucked up the chimney. And you will have
to re-inflate them far too often.

I you're closing a fireplace (mad option IMO) then do so properly. Have
it capped with adequate ventilation arranged at the time and have the
opening blocked off.

Better IMO to have enclosed stoves or to have an inset with a glazed
door. That stops the cats getting into the fireplace and also makes use
of them.