2004 Citroen C8 transmission not driving

Today a number of times I got a "Speed too high" warning message showing on the Screen in the middle of the Dash. At any time I was only doing 40 > 50 MPH.

Then I noticed that the Engine was revving if I accelerated but the Car would take time to pick up speed similar to a "Slipping Clutch" on a Manual Car. A few Miles later the Car was down to a max of 15 > 20 MPH. Because I was holding up Traffic I pulled in on the L/h side and allowed others to pass. Unfortunately I had pulled in on a slight incline and when I went to take off again with the Car in "D" the Engine revved but there was no movement forward whatsoever. At this point in time "Reverse" was still working and I was able to reverse 100yds to an offroad space. We then towed the Car (in Neutral but with the Engine running for the benefit of P S etc) for circa 4 Miles and when we got back to base the Car would rev without any effort to move in either "D" or "R",

Any suggestions - does the Auto Trans need to be overhauled?