When you say you couldn't remove the ECU you meant the alarm ECU?......I have a similar problem with my disco...but...wait for it...I unplug the alarm ECU..& the ECM & it starts & runs fine...Work that out!.....I don't have any speedo, central locking, & some of my interior lights aren't working. I am have changed the alarm ECU ..just the same. I am now waiting for an MFU (Multi function Unit) to come & hopefully that will be my miracle cure!
I unplugged the cable that goes to the transfer box for the speedo (No voltage, even turning the wheel)..voltage to the interior fusebox but none to the speedo connector on the dash....treble checked all fuses & they are fine.