"Andrew Gabriel" wrote in message
...

[...] One other factor is that I don't
want to rule out the possibility of using it as a fireplace sometime
in the future, so I guess a layer of expanded polystyrene under
the screed is ruled out. Is there anything similar suitable for
high temperatures?

Any suggestions on construction methods, including suggestions
for the mix and depth of the floor?

How about a vermiculite mortar (as used for filling in behind firebacks)?
You can still buy 'micafil' for this purpose, or at least you could in '96
when I did my fireplace. The mix is about 1:6 OPC:micafil (no sand), with
just enough water to make it workable. I'd guess about a 150mm thickness
would be OK, with (say) a 40-50mm sand & cement screed on top to get a
smooth surface.

--
Andy