So it should look something like this:

http://www.the-wooden-hill-company.co.uk/karina/karina_space_saver_staircase.htm

when it's finished ?

Hi BigWallop,

Yep - but not the space saver style - this model...

http://www.the-wooden-hill-company.c..._staircase.htm

I just got a post this evening from Bill Howe who sells the stairs in
the UK giving the following information:

The weight of a 14 riser Kompact 89 is listed as 288kgs., if you are
thinking of putting an outside handrail on it would add extra weight,
so I would suggest 300kgs would be a nice round figure to work on.

so, am I close to understanding what specification the beam should be?

I've just had a nasty shock and realised I have to pay even more for
my building control fee than I though because the loft room will be
over 10m2 floor area. It works out at 150 ish for plan fee and 350 ish
for inspection fees. This makes it even less likely that I can afford
a structural engineers report!

Philip