My 2003 S-Type has a gear selector knob which freely spins... I cannot remove the knob from the shaft but it is a little annoying. Is there an allen key that tightens this down?

 

I believe it simply unscrews. Is it the OEM knob?

 

It appears to be OEM... I have tried turning it to tighten but it just spins. Like I said, it doesn't come off, just spins.

 

The metal threaded nut embedded in the gear knob must have released. If so, your knob is just spinning around the threaded nut. I would cover the knob with a rag (or better yet, a piece of rubber such as a jar-lid opener), clamp onto it with a big set of vice-grip pliers, and pull straight up with all my might. If the knob pops off, you can then unscrew the threaded nut using your pliers....

Once you have the pieces in your hand, you may be able to glue the threaded nut back inside the knob and re-install it....

 

To add to the previous advice, the metal piece (with the two wrench flats) at the base of the knob is actually a separate lock nut. You have to turn this nut down and away from the knob to be able to unthread the knob itself from the shaft. If you can get lucky and can break the locknut free (try quickly tapping on the end of the wrench with a mallet), you may be able to then unthread the knob with its metal insert. From there, you'd have a little better access to figure out how to resecure the metal insert inside the wooden portion.

Hope this helps.