We will be in Lauterbrunnen for an anniversary.
What is the best restaurant there or is there a special place in the vicinity which anybody would recommend?

I found that the Hotel Silberhorn had a good restaurant. There may be others in Wengen (short train ride).

I can second the Silberhorn. We stayed there for three nights a few years ago and the meals were superb! I dream of going back there.

The Silberhorn has good food. The last time I ate there, however, smoking was permitted in the restaurant.

For that reason I usually prefer to eat on the terrace of one the other hotels like the Oberlander.

Wengen has some good hotel restaurants. The trains run fairly late up and down the mountain although after 6 pm the schedule is a little less frequent.
The last trip down is a little after midnight so you don't run much of a risk of being stranded.

We stayed at the Hotel Oberland and loved the food. The owner is the chef and we found the menu to be quite varied and good. As Bob says, the Oberland has a nice terrace so that would be an option if the weather isn't bad.