Remember that not all CCCUs have full functionality. All UK CCCUs are of the highest spec, but many CCCUs in Ireland and Central Europe don't have the complete range of features that UK owners are all used to. If the CCCU keeps rejecting its coding, that's possibly because it doesn't have the ability to implement the requested change.
Specifically, not all CCCUs have the an integrated alarm system. 2001 A2s were fitted with extremely unrelaible CCCUs - I've changed more of them than I can possibly remember. It is therefore entirely plausible that the CCCU has been changed in the past, but that the person who changed it fitted a CCCU that doesn't have all the required features. This would explain why the alarm doesn't set, why the sounder doesn't beep, why it won't accept the +1024 coding change, etc, etc.
Alternatively, a replacement CCCU with the required functionality has been fitted, but this functionality isn't coded. For instance, the coding to activate the alarm system is a prerequisite of any further alarm-related coding being accepted.
An example: The alarm system is activated using the 4th coding bit (you add or subtract 8 from the code). The beep-on-lock function is activated using the 11th coding bit (you add or subtract 1024 from the code). But you cannot activate bit 11 unless bit 4 is active.
Cheers,
Tom
Oh.. i see the later date on my CCUU than the car is. So it can be changed.
Maybe you know the functionality of this CCCU?