This is a similar problem I have with my 450 SL. I find that disengaging the shifter from park and then putting it back into park, with a little persuasion will let it start.

The start-in-gear protection switch is usually at fault. Also look at your battery leads, ground straps, dont forget the positive lead at the starter motor too. All these items can, and do, get loose, or corroded. I would look at these items if the shifter dosen't do it for you.