In article ,
" writes:
My house was built with a flat roof. The previous owners had pitched
roof constructed on top of it so that when you enter the roof space
you can walk anywhere on the original bitumen & gravel finish. We
have broken through the old roof to fit ventilation trunking for a
bathroom extractor and I see that the old roof is made up of a
boarding material about 50-75 mms thick which seems to have been made
with a straw-like material.

Strawboard. My parents had a flat roof extension made with this.
They got 35 years out of it, which was twice it's normal life.
It's usually killed by condensation causing the straw to rot,
which causes the felt surface to sag, collect water, and and when
it eventually splits, feeds more water into the strawboard.

I don't think that the old roof gives much insulation and it may be a
good idea to upgrade it.

Could I get a cavity wall insulation co. to blow some sort of flaked
material into the void above the bedroom ceilings and below the old
roof? How might they do it?
Is there any way that this could be a diy job?

You could simply unroll normal loft insulation over the old roof.
That would keep the old roof warm and hence without condensation.
There should have been ventilation in the old roof, and if that
was extensive (which it usually wasn't), it might defeat any
insulation on top of the old roof.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]