I know its melting point is about half that of steel, but it's still 660 degrees and I've a piece of aluminium (3mm) that I could use for the register plate, but if I have to use steel I'd have to buy it!

the main thing is that it is not combustible, so it ticks that box, & much easier to work with

Aluminium blocks of petrol engines seem to cope OK.

My guess, and it's only a guess, is that it wll be seriously affected by the heat. What you need is the opinion of a proper scientific type person, not somebody witha mutual interest in bikes!

You could just make one and try it. Tightwad 😉

The Part J document is linked here…. http://www.stovesonline.co.uk/stove_building_regulations.html Don't have time to read it myself, but it must be in there somewhere…..

As far as I can see it doesn't actually state exactly what should be used other than fire-resistant.
I know I can use steel and Masterboard but have been put off the latter by a couple of people stating that it just doesn't last as long as steel. I'll see if I can pick up a piece of steel from somewhere locally.