Stoves Online has a useful page on register plates, and Bowyer at AirgunBBS.com offers some sensible (but unqualified) advice.
The register plate we had installed for us (by a HETAS-certified installer) was a sheet of fireboard, and wasn't sealed fully around the edges. There was also very little seal around the stove flue pipe. Our sweep wouldn't go near it (until we'd got our installer back in) for fear of smashing up the fireboard with his brush.