I have to say I believe what you need to do is just obtain and enter the code rather than decode. I think decoding is what is required if it has had the wrong code entered too many times.
So what you do need to do, when you get the code by providing the model and serial numbers (for which you need to get the radio out of the dash, for which you need extractor tools), is enter it correctly and all should be well.
If for any reason the code isnt entered correctly and it does lock out, then you will need it "decoded" by a specialist (the Citroen dealer or an auto electrics shop), who will charge you handsomely for their services! So no I do not know how to decode, nor do I have the equipment required.
However, my original comment stands: telling people it is a Saxo Desire still does not provide the information I said you need to provide. You can form your own opinion about my postings.