Taking the above comments into consideration you could use something
like 150x75 C24 so that saves you 70mm as well.

Then then, the 76mm RSJ that it later came up with saves me a whopping 120mm
(the existing 4" joists limit the saving to this value). This is no idle
matter. Headroom is one of the possible no-go reasons on the conversion (the
other main worry being foundations). Every 10mm is an enormous saving.

Because it is a pain to work with. When you want to trim a bit of here
or there it is not just a quick cut with a saw. Fixing other beams to
stringers is not as simple as nailing on a joist hanger with square
twist nails!

OK. How do you hang a steel joist? Does it lie on top, is it bolted down,
can you just use joist hangers a la timber.

Also not as easy if you need to drill a hole through the
centre of a joist for a pipe or wire run.

The 76mm RSJ gives me 24mm between the laths of the old ceiling and the
bottom of the steel, assuming the top of the steel is flush with the
existing ceiling joists.

True - but even 10mm is not that bouncy. If you need to go to steel at
all - then looking at flitch beams first, gives more "workability".

Well, the flitch beam gave me 150mm, which is a vast improvement. I wonder
if you can convert a steel beam into a "decorative" flitch by calculating it
entirely on steel strength with some wood bolted on to have something to
screw to?

of the advantages of working with wood, and you are still in the bounds
of manually handling the beams without needing a crane (you can lug the
three bits up a ladder separately and assemble the beam in the loft).

I'm sure it wouldn't take long to get 20 4m steel beams into position, would
it? Crane hire for a day can't be that bad. Front access is excellent,
unlike the rear.

Timber will cost you as much in all likelihood. The floor joists cost me
something in the order of 500 quid for the wood, and 400 for the flitch
plates (I used four) plus all the hangers, bolts, dogs etc. Having said
that I saved on the cost of a crane!

Bloody hell. I though the wood would be a fraction of that. I never bothered
to calculate it. How much can a little crane or hoist cost to hire for the
day? 250 quid?

Also, I bet the 76x76x12 RSJ is probably not much more than half the
127x75x13 UB in weight and cost.

Christian.