We had problems with our chimney and tried different cowls,none of which were any good.Plenty ventilation in the room.
We ended up buying an electric fan on the net from england.It is wired up to a speed switch,so you can adjust the draught to suit depending on the day.It is agood job although on full speed,a lot of the heat can go up the chimney.
If my memory is right I think it cost approx £700 stg.Google electric chimney fans