AAXD..after giving your problem more thought, i'm starting to think the engine ecu might be the problem after all. The engine ecu on the 97 disco is located in the engine compartment, along the back edge of the passenger fender, where the fender meets the firewall(if my memory is correct). These are very prone to water ingress. The ecu should have a plastic water shedder cover over it, held on by two fir tree clip type fasteners. Pop these off, and you'll see the top of the ecu. Three plugs on top, and two small bolts,(8 mm) and you can slide it up and out. You should have no problems removing the connectors and removing the ecu , as long as you do this with the battery disconnected, there will be no ill effects. If your brave enough, you can remove the torx bolts that hold the ecu side covers on..then you can inspect the circuit bords for any signs of water or long term corrosion. PLEASE BE WARNED..typically ecu cover removal is done with an anti-static mat and grounding strap, as the electrics can be damaged by static discharge, although this is uncommon(use of the mat, and damage caused by not using it are both uncommon)
The tach not reading correctly, or sporadically, is usually an indication of the alternator beginning to fail. The tach signal comes directly from the alt, and this is pretty common.
You're right, the speedo bouncing around is , most likely, the speed sensor. The speed signal goes directly from the sensor to the instrument cluster..from there, it goes to feed a couple other units, one being the cruise ecu, and this would explain the cruise dropping out on you.
It's unlikely that the tach and speedo issues are the cause of your no start.
You say that you have neither spark or fuel..this is a pretty good indication of some type of large scale failure.The fuel pump should run and prime the system when you turn the key to position 2, regardless of crankshaft position sensor.
The gems ecu serves as both the engine ecm, and the security ecu. Both operations occur in the gems unit.
A quick check of the gems enabling a start condition is to do this....let the car sit, with the key off, for at least ten minutes....get in the car and turn the key to position 2...take note of the check engine light on the dash ...if this illuminates, it usually indicates that the security is ok and enabled..if you get no check engine light illumination, this usually means the security system has recieved the correct code.
good luck, and let us know how you make out