I have just had a detached garage built with a room above and need
to fireproof the garage ceiling and stairs. I was told by the
buildings inspector to use two layers of plasterboard, but will one
layer of Fireline plasterboard do the same job?

no, he means 2 layers of Fireline

Are you sure?

yes, according to our local BCO if there's a habitable room over a
garage there has to be 2 layers of fireline (fire resistant)
plasterboard

LJ