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 http://www.chimneyballoon.us

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? http://www.chimneyballoon.us/Chimneyballoonsizing.html

or does soemone else know of a more detailed chimney diagram we can
refer Dave to look at?