From Elise FAQ
S1 Locking Wheel Nuts
The locking wheel nut remover is found under the bonnet, and comes inside a black plastic tube with a red bung in the end, however you will also need the tube so remove this as an item when you want to remove a wheel.
Take the bung out of the end and you should find the locking nut remover. There is a number etched into the end of this nut, and it is this number that you will need to quote at a Rover dealer to get a replacement should you lose this tool.
To use the removal tool, first identify which of the 4 nuts is the locking nut (this is because if you get this wrong it can lead to you needing a new black plastic tube). Now, place the black plastic tube into the relevant hole as you are endeavouring to use it to remove the slotted cover from the locking nut (and hence you'll need to insert it with the end, opposite where the red plug was, going in first). Push this on until it clicks over the cover (you may need to use your palm), and then remove the tube and with it should come that push-fit cover.
Next, take the metal remover nut, and place it in your socket that you would normally use to remove any of the other wheel nuts. Now place the socket and nut over the locking nut and slowly turn it until the slot engages. You're now ready to remove the locking wheelnut.
When replacing the locking wheelnut, after you have tightened it to the required 85nM (using a torque wrench of course) then you will need to push the slotted cover back onto the locking nut firmly. Once again, save yourself from looking foolish by making sure you push the cover onto the correct nut!