Ok, so I thought i would just write a brief note on the question above which seems to have come may way a few times over the past 8 months.
Firstly, is there really an appropriate time to leave the dinner party?
I don't think there is.. I think you leave when you feel you have been there long enough. You have had a lovely drink beforehand, a delicious dinner from the host, you are grateful for all they have put into the evening and you have talked to the other guests and the hosts. You are tired and it is time to go home. That is when you make your excuses and leave.
Let me just point one thing out. Having been the drunk and the sober for a very long time guest, if everyone was sober at these events, most of the time, the event would be over by 10 and MOST of the time time 50% of these social engagements would not even take place. So enjoy yourselves. Say thank you and leave when it feels right to you.