I've killed darth nihlius and set 3 of the 4 torpedos. I've visited all 4 torpedo sites a number of times, but had no luck. It still says I need to find another site.

 

I take it I've hit a bug?

 

Mark

There are four spots. Did you find all of them. You should get a cutscene explaining the situation with the fourth.

You can get the 4th from the missile bay, can't remember how exactly, but its pretty simple to figure out