[i]Now we're talking.[/i] 

It's unlikely the Void Stalkers are on Ullanor Prime. Considering Urlakk Urg was hoping to move on to another sector I think it's safe to assume he had a fleet of Roks and Ork ships in the system, and the means to build them. Who wants to help the Void Stalkers destroy the shipyards? 

EDIT: Actually, Gabriel and the Sixth Chapter (with a few Unseen) could be on Ullanor Prime.