On a quick check a 90x90x3.6mm hot-finished SHS (square hollow section)
Grade S355 (Grade 43) steel would do the job and weighs only 9.6kg/m.

I've just tried it in SuperBeam, and I'm happy with that! At 40kg a beam,
you could carry them up the stairs with a few mates! I bet a square section
would be really easy to work with, too. It also reduces worries in terms of
foundation strength. I'd need about 20 of these beams and 800kg doesn't
sound that scary.

I had thought about the self drilling screws (I'm using them in my
conservatory), but assumed that the beam sections would be too thick for
them. My only concern is a possible point load when it comes to redesigning
the roof bracing that might require a vertical column to support a purlin.
However, this could easily be accomodated by hanging a much larger beam at
that point and not bothering to attach the floorboards to that one. (i.e. a
90x90x5 SHS).

I have no idea of how to calculate the roof structure to determine the
magnitude of the point load. For all I know, the purlin will just be
replaced by a steel beam itself and not require further bracing to the
floor.

You'd need to check out the fireproofing carefully: I suspect it would
come to intumescent coating by the supplier with any damage made good
on site.

Any idea how much an intumescent coating would cost? Is it significantly
more (i.e. triple). I bet the 90x90x3.6 SHS would cost peanuts uncoated.

Christian.