In message , alan_m
writes
On 23/12/2020 11:48, wrote:

Thanks, but I'd ruled-out a DIY version of those because of the
problems of storing a big sooty thing, and crud falling down when
it's pulled from the chimney

Irrespective of any mode of blocking you are going to get the crud that
has fallen on the blocking mechanism dropping down when opening up
again, otherwise where do you expect this crud to disappear to?

Even with a bespoke fixed design consider how it is to be cleaned.

I think they centre pivot so the crud slides off when opened.

The fixing trick is knowing that you have to take out and replace some
brick above your fire opening.

--
Tim Lamb