I have attached an image to illustrate the bump that is preventing the arm assembly from moving downwards. I have tried lifting the assembly, twisting the part closest to the bump but there's so little play that it won't budge.
update: Sad but true, out came the screwdriver to unscrew the arms (just above the start of the arrow), tilted it inwards and then it moved into place.
I'm not going to say how old I am, but at my age you'd think this wouldn't be so hard!