Not sure if this will help anyone else, but I found out some things on Friday.
Ends up I did have the code wrong, my bad.
However it also ends up that Clarion weren't entirely honest, and if you disconnect and reconnect the power enough times (3-4?) it reverts from the locked screen to give you the keypad. You then get three cracks to guess the code before you lock it again, so have to goes, turn the car off, back on, another two goes, etc until you get it right.
Sounds like a pain, and probably easier not to use that feature. I've now recorded my code in Evernote so I can put it in later if needed, and hopefully this helps someone else.
YOU DO NOT NEED TO SEND IT BACK TO CLARION! They replace an IC in the deck if you need the password reset. If you just need it unbricked to have another crack, you can do it yourself, without sending it back to Clarion.