Which is India's national language?

A bunch of our friends got together for a bbq party yesterday evening. I had some really good food followed by an ice cream. Once I am full, I start poking the weak points on other people till they piss off or I get pissed off! (sometimes both!)
This time I picked a nice young guy (mallu guy brought up in Chennai) who happened to say that he doesn't know how to write in malayalam or tamil. Now before I could go full fledge on this 'weakness' he defended by saying - "but I know Hindi - India's national language".
Hm.. interesting.. Hindi is our national language? Since when? The nice guy's nice wife supported quoting the 5th standard hindi text book - "Hindi hamari rashtra bhasha hei \| Hindi ek saral our aasaan bhasha hei\|".
Now another not-so-nice guy (a mallu guy brought up in mallu land) who argues for anything and everything emerges from the group (that is still enjoying the ice cream) and goes "yeah, Hindi is THE national language".
I spent my following valuable 30 minutes to 'educate' them on the fact that Republic of India does NOT recognize any language as a "National Language". Instead of proving them wrong by asking them to find one official document that states so, I took the stupid route of proving multiple times that Hindi is not national language - Interestingly Constitution of India is silent on that fact which gave them the advantage of the pursuiting the argument.
I left finally by agreeing to disagree.
Now the facts -
National language is more of a symbolic status, a privileged position that we assign to a language. I or you don't make that choice. Constitution of India makes that choice.
http://lawmin.nic.in/coi/coiason29july08.pdf - Constitution of India makes a choice to call English and Hindi (both) as 'Official languages of India'. That is no privleged status. Official language is in which Central departments and Judiciary has to communicate.
http://en.wikipedia.org/wiki/Languages_of_India - (Yeah, not a very authentic source. But believe me, nobody has contested the fact given here though) - says that "India does not have any national language".
I didn't see a single official government circular or a constitutional reference or amendment that says Hindi is our national language or otherwise. Trust me, I even visited a bunch of Indian embassy websites which portrays National bird, anthem, animal.. but not a word about language.

And that proves that the question "Which is India's national language" does not stand itself. The answer (either way) does not have a legal/constitutional validity. It is as good as a question "which is the most beautiful language in India?". You can have an answer, but don't insist that people have to agree.