I have now managed to do this. The catches have to be pushed straight up as stated in the manual but they are very stiff, may get easier with use, and have to be pushed up quite a way to clear the up-stands on the top of the glove box.
There is a guide bar on the left side of the box which promptly falls apart as the box drops down but it is easy to put back together, and it is important to get it right before pushing the glove box back into its normal position.
To drop the glove box I found it easiest to be knelt outside the passenger door and work from there, it is very difficult to get the right angles from inside the car.