[K.S.H]

Just on with this myself but there is a firm who advertise in the Telegraph and do it for £280 IIRC, they don't do the walls though, you get loft ladder, floor and a light.
I'd a lot more to do, taking a structure down that was in the loft and removing all the loose insulation as well as the muck that fell in from when we had the house re-roofed, put the hatch in with foldable ladder, strenghtened joists, floored and walled, just finishing it off now