It's not under the dashboard, it's in the engine bay- inside the passenger side firewall, directly behind the battery connection post. What people refer to as the "ECU" is actually called the ECM, which is the heart of the DME (Digital Motor Electronics). In the Bentley, refer to sections 130-26 and 130-27: "Engine control module (ECM), accessing."
There's a picture of it in 130-27, figure #42.
Funny- I don't call it ECM either; in my mind, ECM stands for Electronic Countermeasures (a military term)