Traditionally Agas would have a cast iron flue with no boarding around
them - you know it's hot and don't touch it. Nowadays it's also common
to use insulated stainless steel flue pipe (more with woodburners than
Agas), which considerably reduces the requirement for fireproof
materials close to the flue (it's expensive though). But if you have to
board around a hot, uninsulated flue, I'd look at the various cement
boards available (similar to plasterboard type material). What
supported the old asbestos boarding - wasn't fixed to wooden studding
was it? Building regs have things to say about minimum distances
between flues and non-fireproof materials.