Christian McArdle wrote:

Any ideas of what this might be/sensible questions to ask before I call
in the professionals so I don't get ripped off, please?

Are the voids properly ventilated? A bodge job might have left the voids
totally sealed, which will lead to problems. You need lots of ventilation at
the eaves, a gap between the room and the roof covering and ridge
ventilation. Is there any evidence of this?

It's hard to say. The voids have been insulated to the gills and then
boarded over on the floor and internal walls, and that brittle
tar-papery-stuff on the roof. I suspect that the insulation has been
shoved into every possible oriface; it was definitely sauna-like when I
stuck my head in there during the hot weather. I'm going up there this
weekend to dig out the eaves, at the least. Viewed from the outside
there are modern roof vents, but it's hard to say if they're venting the
void space or going between the floorboards.

Inside, the roof timbers are starting to exhibit traces of something
unpleasant; there are "tide marks", and inside these are scattered blobs
of a white fluff, like a scattering of icing sugar, and very similar to
some mildewy-like stuff I once had on some old leather.

That sounds very like rising damp. Salts are drawn up by capillary action
and when the water evaporates crystals form. It was in my childhood home.
If that is in your roofspace it is hardly being drawn from the ground, so is
there anywhere moisture could be getting in? Could fascia or soffits be
damaged, or even a blocked guttering?