What's heatproof, waterproof, and acts as an insulator?

Well Asbestolux still seems to be available - http://www.fluetech.co.uk/store/asbestolux-board-1200mm-x-595mm-p-5.html - so must be asbestos free I assume the name was kept for familiarity.

I am amazed! I searched the website for a safety data or technical data sheet which would reveal the content in detail but coukdn't find one. It certainly will cause a lot of questions to be asked if a business or householder gets to see that a product with that name is being installed at their premises or home. Weird or what?
 
a quick check that some heat resistant silicones are brittle....

No more than the alternatives. I used a 600deg silicone just as I described and produced a flexible, heat resistant donut. I used this as the spacer and sealed it weathertight with more HR mastic. Has been sealing my old Taylor's flue for three years now.
 
1-1200 deg silicone 9quid a tube.2-verlicumite board 20quid a sheet 25mm thick both from hotline chimneys online site.3-hardibacker 12 quid a sheet from any builders merchants, I have lots of offcuts you could have free but it is heavy and postage is not cheap.
 
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