A 6310i certainly doesnt work in my 6210 cradle
Chas, The PSE on your W220 is located under the drivers side rear seat. I bought mine from www.audiocomp.de. Although this site is in German, email the webmaster and they will get back to you.
It is fairly straightford to swap over the PSE. Part number is on this thread:
http://forums.mercedesclub.org.uk/showthread.php?t=6744&highlight=nokia+6310+PSE