Remove 4 screws in the cover beneath where the optical sensor was.

Remove two screws at the inboard underside ends of the front arms.

Locate two screws retaining the front arms, accessible from deep within the bttery comparment. These have 2.5mm hex heads. Loosen them by a few turns only. You will most likely need a screwdriver extension, or a 2.5mm allen key. Don't remove them.

You should now be able to pull each of the front arms out by a millimetre or so enabling you to remove the cover.

Feed the ribbon cable back through the slot in the cover as you remove it.

On reassembly, don't forget to feed the ribbon cable back through the slot.