Alternatives to asbestos flame-proof sheet

I need to create a flame barrier which must be thin (5mm approx) and fairly strong. Asbestos sheet would have been ideal if only it was still available.
What I need it for is to block a chimney off with it, diverting the heat and fumes off to another flue. It'll be located fairly close above - but not actually touching - the flames coming from a 'living flame' gas fire immediately underneath.
Anyone know of a good substitute for asbestos sheet and where to obtain it?
Thank you, MJ
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PS By "sheet", I meant stiff board rather than felexible fabric...
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Chris Oates wrote:

Multiboard
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M. Jakeman wrote:

We got some stuff that is very similar from our local DIY shop. I think it's actually concrete.
Cheers Tim
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Thanks to everyone for the help. Anyone know where I can optain any of the suggested materials? B&Q doesn't seem to stock it.
Jake
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M. Jakeman wrote:

Try "Sheffield Insulation" http://www.sheffins.co.uk ??? We get quite a bit of our refractory sheet from them,
Steve
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