How do you replace a vitra retro close toilet seat?

It's doing my head in! There is no access behind the seat which is integral part of the toilet unit and if i unscrew completely the old unit, the 2 nuts will fall at the bottom of the unit and i have no clue as to how i could re-insert 2 new nuts AND screw the new toilet seat!
It's worse than the Krypton factor!!