I've cracked all of the 3 codes that were mentioned by the game's author (one from the dust cover and two from the actual game book) to obtain the additional "secret" content.
Can I check with the author as to whether there is a fourth "code" to be cracked, specifically "/R"? I note this goes to a page asking for a password. That being said, this page may well serve other purposes that are not related to this game :-)
I will be setting off to TEL 022 tomorrow - so wish me luck!
Hazmos la vista amigos ;-)