How can engine mgt light problem on Ford Mondeo 2.0 DCi Automatic be fixed?

I have owned this car since July 2009 and on a number of occasions since, I have "lost" the speedo and milometer, and the engine management light has come on. The car has otherwise driven ok.
This is an intermittant problem which has been temporarily "cured" on a number of occasions by my having a diagnostic test and recoding done. However, I am concerned, knowing that it will recur. The last advice that I have had is that there is a problem with a sensor, or possibly sensors, which are situated within the transmission and that this will require work costing in excess of one thousand pounds in order to accurately identify and correct the problem. Does this diagnosis seem to be right, or might there be a simpler and less expensive solution?