The spider unit ( so called because it has 8 wires connected to it ) causes problems on the 300 Tdi - either preventing them from starting at random or cutting out , again at random . One of the circuits it controls is the feed to the pump solenoid - no feed means no start or if running engine cuts out . It also controls (AFAIK) the feed to the starter solenoid which again prevents starting.
You can bypass it by running a wire from the white wire on the ignition switch to the pump solenoid - so engine will stop when you switch off .
You could , as a trial , run a wire from battery positive to the solenoid and see if the fault is cured -
but you have to disconnect the wire to stop the engine .
It is a very annoying type of fault which , as you say , makes you think twice about using the Disco at all . And it could be dangerous if you stalled halfway across a busy road - as happened a friend of mine .
There is a good post on this subject - when I find it I will post it up for you.
Found it . Shows how to get to the spider and repair it if you want or fit the bypass harness supplied by LR - part no AMR 4956.
The Definitive Alarm Spider Post - LR4x4 - The Land Rover Forum