Disable Dolby digital in audio settings. Change it to PCM in audio settings of the set top box. Then, the volume control will work. It will work even without this change on SD channels. Dolby digital does not have the facility of volume control. It has to be done by the audio processor. In this case, it is done by the TV.
Although, I recommend you to program your Tata sky remote to control the TV volume (and keep DD on STB) as the sound quality on DD vs PCM is night and day.