You'll feel the screw clicking against the captive nut once it's free. If you must remove it, a thin nose pliers will do it.
If the leading edge of the light unit won't come free from the car easily, spray some lube in to help the pegs release. They're at 90 degrees to the cluster, and need to come out sideways, if that makes any sense. Go canny, otherwise there's a risk of cracking the cluster. When refitting, get your screwdriver / socket onto the bolt before you push it back into place. This will help stop the wretched thing dropping into the cavity. Fit the trailing edge into place, then push the pegs into their sockets.
If the bolt does decide to make a bid for freedom (and I suspect most of these cars have a pile of suitable bolts lurking in their innards!!) then a suitable replacement was (for me anyway) easy to find in my box of spare nuts and bolts. Can't remember the spec I'm afraid. A tip for replacing if it has been taken completely off the car is to push it thro a short length of 1 cm masking tape, or electrical tape such that the sticky side faces. Now put the screwdriver or socket in place, and bring the tape up the shaft so that the bolt is securely held.
Now you've got the bolt secured on the driver, you can insert it into the access hole and through the bodywork without it falling off the driver. When you've got the thread started, just rip the tape back.
Final tip - go canny with the plastic cover for the access hole in the trim. Again, most cars will have a few in their innards. Both mine have been replaced with a suitable rubber grommet! So, as you can tell, I've learned a few lessons along the way. My old Mondeo didn't require any of this faffing around - a couple of thumb screws and out came the back of the light unit!
At max, this is a 10 min job - and fairly satisfying not to be paying stupid amounts on money to a garage to have it done!
