Actually this is only the fourth time, and in reality would seem to be the first genuine and sustainable attempt. The previous cases were:
1. American to Chicago. Far too early in STN's development, and politically motivated.
2. American to JFK. Clearly a spoiler for the premium carriers. Binned almost overnight after the collapse of Eos.
3. PIA to Karachi. Ceased after PIA's aircraft were banned from EU airspace.
Continental operated to Newark of course, but they were not at Heathrow at that time. This service did well, but was canned in the immediate aftermath of 9/11 and the collapse in trans-Atlantic air travel.
I am in no doubt EK will do well, and I believe a second daily service will come forward within a couple of years.