Actually fellas, it sounds to me like Dave is refering to his fireplace chimney damper. Often times they are completly removable from the flue, especially in British style fireplaces. If you use the fireplace rarely, It is a better option to go with a tighter fitting damper like a chimney balloon than getting a replacement metal damper. This style of damper can be found at
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If you use the fireplace weekly or daily then you will want to look for the appropriate metal damper replacement or go with a chimney top damper. These are generally installed by a professional.
By the way, to clear up any confusion in what you are referring to Dave, can you please identify on one of these 4 fireplace diagrams where this 10" cover is at?
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or does soemone else know of a more detailed chimney diagram we can refer Dave to look at?