Are they the original (Mordaunt short built) Mk1 drivers or the Mk 2 (Naim built) ones?
The Mk 1 phase plugs are glued to the pole piece in the centre of the voice coil. Mk 2 plugs are in 2 pieces, the rear half is screwed, through the rear of the magnet,into place and is unlikely to come loose. The front half is attached to the rear section with double sided sticky tape and a wall plug in matching holes in the centre of the joining faces (that's what the post from iiyama refers to).
If it's Mk 1 type the answer is one small drop of superglue in the middle of the rear face of the phase plug and a thin cylindrical spacer to centre the plug in the voice coil so it clears the voice coil and doesn't rub in operation. The drive unit should be in a horizontal position to do this and DON'T GLUE THE VOICE COIL TO THE PLUG!
