Thank you all for your input, it is most insightful and much
appreciated. Thanks loads for the link Bill, this is the closest thing
to a supply have.

After a bit of research, I came across a few suppliers who are still
selling products which appear identical(ish) to my elusive container.
This one:

http://www.esedirect.co.uk/store.asp?item=CA00001

calls this material 'Resin Bonded Fibreboard' in the catalogue. I've
also found a couple of bespoke container makers who appear to use the
same stuff.

The stuff from RH Nuttall does appear to be a bit different as it is a
vulcanised product but hopefully with a bit of experimentation I'll
achieve the results I'm after. I only hope they can supply me with
small enough amounts.

Chris