Which model Disco? People should ALWAYS say that at the start.
DRAIN IT RIGHT OUT immediately.
Does the engine even TURN? If it does, do NOT try to start it. Take the wire off the solenoid stopper if it has one. Take off the oil filter and drain it out, then fill it with kerosene and shake it about, and tip it out again, then put it back on the engine.
Now put the sump plug back and FILL the engine with DIESEL fuel right to the top mark on the dipstick. Now TOW the car for a few minutes IN GEAR (use 3rd or 4th) but do NOT start it up. Now drain all the wet diesel out again, and refill with ordinary engine oil. Fit a new oil filter. Tow it in gear again for a few minutes , to flush good engine oil everywhere inside the motor.
PRAY.
See if it will start up.
Been there, done that, long time ago, customer's car, engine survived without a problem.
CharlesY