Hi Daz,
I had exactly the same problem.
I recently used DriverMax to update all my drivers unfortunately the SigmaTel update stopped my mic working on the rear input socket.
I even tried rolling back the drivers.
I went to the Dell site put in my tag code and went to the drivers section. Listed was an update for the SigmaTel which I downloaded and installed. It asked if I wanted to overwrite the existing .exe when it was unzipping, I said yes. I then installed the new drivers.
Mic worked straight away on the rear socket.
Good luck,
Booga
January 2009