When you open the fuel filler cap, take a moment to look inside the gap behind the cap. If it looks anything like the below, it is time to replace it.
Looks naff. Time to replace.
Open the fuel filler cap, unscrew the cap and using a T25 torx bit, remove the six screws that is there.
You'll see that the bottom screw is a lot shorter than the others. That is because it doesn't appear to do anything apart from complete the pattern.
Pull the fuel filler lid off and put aside. You'll see two further T25 screws. These do not need to be removed since they don't hold the sleeve on but if it doesn't come off in the next step, unscrew the offending screw and then screw it back in.
Grip onto the sleeve and pull (not much force is required). Take a moment to compare the old and new sleeve and be happy knowing that it is being changed.
Before screwing the new sleeve in, check that there is a slit for the pin to lock the filler cap. Although it doesn't look like it, there is a split in it.
Put the new sleeve into place and start screwing the T25 screws back in. Do the top first and work your way down.
Test that locking the filler cap works and then admire.