by the time the last episode aired, the show had probably the same % of the original actors as Primeval now has [as of episode 3.04]...if not an even smaller percentage than Primeval.
although I think a fourth season would have been fantastic!
...even if for no other reason than to have a return of DSV's greatest discoveries ever:
1) the face of the Chao-Dai Confederacy
(portrayed by Julia Nickson-Soul)
2) the leader of the G.E.L.F.s, Maria
(portrayed by Sam Jenkins:
3) Commander Ford's lady friend from Abadon