Yep there was indeed a locking tab underneath, god only knows how I missed them!!!
Another question if I may, successfully removed the OEM radio and have removed the warning light and placed in the new fascia, seem to have a problem removing the air vents though, 2 screws have been removed although not sure what is still holding them in place??
Ah I had this same problem but I can't remember what it was. There was another screw or a locking tab somewhere that I missed... sorry I know that's probably not very helpful but I do remember being stumped at the same point at first!