Not being facetious, but it depends on the instructions. One of those messy areas where we all develop an opinion which everyone else will disagree with I expect.
Some of the old ones didn't even say seal it, whereas the new ones insist on fireproof seal etc.
Corgi say At Risk (iirc) followed by some daft phrase like "unless it's impractical to retrofit a seal", then it's only NCS. It's always practical if you dismantle the ruddy chimney, but I've never done more than stuff Rockwool up there, for an existing installation.
Whenever it has been a problem I've been on site without the instructions so phoneds mfrs, and that's what they say to do.
I recently charged £100 or so because I wouldn't expect to have to carry that stuff, so have to get it, fit it, retest flue etc, taking 2-3 hours altogether.