Where a roofspace is defined as a "non-habitable room", the normal rules and dimensions (Building Regulations) don't apply. You have the choice of a permanent setup where the stair is fixed, or the type that folds up in the roof above the trapdoor.
You'll need plenty of room for a fixed stair in order to be able to walk around it. For the retractable one, just buy one off the shelf and have it fitted by a local handyman. They're pretty easy to fit.
No need for any of those "specialist" companies. They'll only make a (costly) drama out of it.
http://www.screwfix.com/p/1303-050-eurofold-loft-ladder-timber-2-section-12-tread/53939