The Disco1 won't have an OBDII plug.  I think it is OBDI compliant though.
What the factory code reader does is diplay the codes that are logged in your ECU.  It does this by means of a two digit LED display.
This is it
Rover Diagnostic ("Fault") Codes
  12 	Mass airflow (MAF) sensor or MAF sensor circuit
  14 	Coolant temperature sensor (CTS)
  15 	Fuel temperature sensor (FTS)
  17 	Throttle position sensor (TPS)
  18 	Throttle position sensor (TPS)
  19 	Throttle position sensor (TPS)
  21 	Tune resistor (open circuit)
  23 	Fuel system pressure
  25 	Ignition misfire
  28 	Air leak
  29 	Electronic control module (ECM) memory check
  34 	Injector (or its wiring) - cylinder bank "A"
  36 	Injector (or its wiring) - cylinder bank "B"
  40 	Misfire - cylinder bank "A"
  44 	Oxygen sensor - cylinder bank "A"
  45 	Oxygen sensor - cylinder bank "B"
  48 	Idle air control valve (also check the idle speed and road speed sensor)
  50 	Misfire - cylinder bank "B"
  59 	"group fault" (it's either an air leak or a fuel supply problem)
  68 	Vehicle speed sensor (VSS)
  69 	Gear selector switch
  88 	Carbon filter solenoid valve ("purge valve") leak
Note: Fault code "02" will appear after a disconnected ECU is reconnected. Simply switch on the ignition to clear the display. 
http://www.britishv8.org/Articles/Rover-14CUX-EFI.htm