I think you are talking about a restaurant called Si-Fah, the original location was in Wang Burapa. It was something of an institution. I used to go all the time with my grandfather.
When I was a teenager in Bangkok back in the '70's, there was a restaurant that was famous for roast duck served hacked up with white rice. It was frequented by cabinet ministers and cab drivers... does anyone remember what I'm talking about, and is there someplace like this still around? I'm going back in December and already dreaming about the food...tell me where you eat!
The restaurant still exists, the best known location now is in Siam Square. Although I think you may find that your taste has changed. Mine definitely has, I now find the ducks there far too sweet, and not enough meat on the bone. I also now dislike the sweet-ish sauce they douse over the sliced ducks and the rice, finding the taste far too assertive and somewhat muddy the taste of the duck meat itself.
Though I admit I sometimes go there anywhere, more for nostalgic reason than anything else.