Thank you for your reply.
In regards to your query regarding the XAV-622, please contact our helpline “In car lost codes” on this number: 0121 522 6663
Exactly, what 4-digit code? I've never entered one and I've had this thing in my own 2 cars, from new.
I've now finally got through to somebody worth speaking to at Sony and have been given this reply in case anybody else has similar issues:
From the manual
This unit has a security function to discourage theft. Enter a 4-digit number as your security code. Once the security function is activated, you need to enter the security code to turn on the unit after re-installing the unit in another car or charging the battery