Diesel I guess from an implication earlier.
Personally, I wouldnt bother too much. A pro car thief will have it regardless if he wants it. A toe-rag who cant start it is likely not going to just wander away, hes gonna be mad and start destroying it in-situ, either by trying to start it, or just by starting on glass, body panels etc out of frustration. I had an "undrive-awayable" disco (the cylinder head was in the boot) so returned home one day to find every piece of glass and light lens smashed, leading to a write off. Even as a mechanic I have seen a few cars that have had fried ECUs and sensors and/or needed 3 days to rebuild the loom where people have tried to bypass security systems. One of those merely couldnt be started because of a flat battery, yet the final repair bill ran well into 4 figures.
A good loud alarm so you can run out with a tyre lever is best.