how to repair faulty lock onupvc double door?

It sounds as if the problem may not be with the lock but the cylinder, I would suggest you take the cylinder out of the door.With door open unscrew bolt in edge of the the door inline with the cylinder,put key in door turn about 5 degrees and pull out.Once removed check the cam in the centre of the cylinder operates from both ends correctly if not replace with same size cylinder from any good architectural ironmongers

Mark Smith
October 2007