Europlex Unifier - Unknown usercode

I've somehow managed to change the user code on my alarm. I know this seems unlikely but as it's not recognising my user code I assume that's what has happened.
When I first became aware of the issue, it followed a bleary-eyed, early morning, clumsy attempt to unset the code with me apparently entering the wrong code a few times.

I've had an alarm company out and they say that the only option available, is to replace the whole system. This seems a little excessive to me. So I thought I'd just put it out there to see if they may be right.
I appreciate that you don't want there to be backdoors into the system but that doesn't mean there shouldn't be a method (a jumper setting/startup sequence?) to perform a factory reset that will set it back to default settings.

The company who installed the system are no longer in business.