Thanks Tony, done that and manage to unscrew it using 1.2mm screwdriver.
BUT, after took out the small screw from the hole, I tried to slot in the 1.4mm Panerai supplied screwdriver and yes, the tip of the screwdriver can fit into the screw slot.
SO, to avoid any confusion - Panerai supplied 1.4mm screwdriver can be used to unscrew the grub screw. It just that I do it wrong at the first place. Sorry for any confusion caused.