I am not qualified to comment on the directors ruling, but your question raises an interesting point.

I assume that you thought that everyone played that convention, and it was "just bridge", and didn't need an alert. However if the opponents had opened 1 Diamond and you had bid 2NT, what would that have meant?

 Many players have different meanings for that bid, and some players don't use that bid at all. So, "would you have alerted that bid? ". 

If you gave the details of the hand, with the vulnerability, you may get more comments. For example when you competed to 4D, did it occur to you that partner may just have given preference with 2 Diamonds and only 1 Club ? and you were taking a big risk.

The unusual 2NT is alertable I think.  Manual D30.2.1 "All opening bids and overcalls that show two or more suits, even if one of the suits is specified, are by definition conventional." And it's not considered prealerted like a cuebid which must (D30.2.3) be "a bid of any denomination bid by the opponent or of a suit shown by the opponent’s bid."

The ABF regulations are very similar to ours as regards alerting, but they add some useful examples of things that must be alerted, including "Unusual NT bids that show 2-suited hands" (Alert Regulations 5.3.1(b))

Other places follow other regulations.  The ACBL Alert Chart, for instance, says "no alert".  I mention this because I wonder how your opponents came to the opinion that your 2NT showed 20+ and a stop.  I'm assuming they didn't read it in your convention card, nor did they ask you (an action that would certainly have afforded them some protection if they were unsure.)

However there was no alert, and that means misinformation and that means Law 21B3. "When it is too late to change a call and the Director judges that the offending side gained an advantage from the irregularity he awards an ajusted score."  As you describe it, you went light instead of them going light.  So what's the advantage you gained from the irregularity?  Maybe there is some other law involved.  I would have asked the Director what law was being applied and, if it was Law21B3, what advantage was being corrected.

Yes, Giles is right.  I should have said cue bids are "self alerted".

Thanks guys for your responses. Yes Stanley my partner and I did assume the unusual no trump was "just bridge" similar manner to 2 clubs "stamen", so we have learnt from that experience. We play 1D overcall 2NT being both majors so till now would not have alerted it. Me bidding to 4D probably was the key to the directors decision, would not have been able to bid that without the unalerted 2Nt bid. Other tables were making 2S and some 3S. I found the opponent's comments frustrating, to say he thought I had 20+ points and that stopped him from bidding 4S. If he thought I had 20+ points would he have even bid 3S, particularly with no response from his partner!  

I don't think players should EVER be asking about unalerted bids

That would nearly always give the opponents and/or partner  information they aren't entitled to

Except general questions such as "what is the point range" and the like

Or, of course, if the bid is self-alerting

And I believe the same applies for a bid which should be announced but isn't

cheers
Nick