Well, its been a long time since an update...
I still have the bike and I still get that same fault code, however I have after many years of not a lot of riding, sussed how to manage it. Basically, starting from cold, 80% of the time the code 20 will appear and idle is a little rough. All I need to do is wait until temp reaches 101 degrees and the fan kicks in. Turn off, turn back on, and its gone all the time the bike is warm.
I did contact IVAN and the great guy sent me a few old units that he had from part exchangers for the ECU flash. The exchanged item worked for a few months and the fault came back.
Long story short, it has to be a fault of the FCE (although I haven't tried it without...). The bike only throws the fault when its cold and once it reaches the correct temp and the fan kicks in, its gone. All those reading in to the pressure sensors etc I think you are headed down a wrong path in the way that the bike is just getting confused with all the 3rd party kit attached.
Its almost summer here in the UK, happy riding!