Just putting the wiring loom back on my td5 after a head job..

The plug that goes onto the injector wiring harness is hanging on by one retainer clip (side plastic clip). So if this snaps I'm guessing the car will stop.

Can you buy these plugs individually or do you need to buy a whole new engine wiring harness?

you can't find those plugs as spare... unless you get it from scrapyard, though if you can afford a new engine harness is a good move anyway cos there are all kind of weak connections in an old one... i replaced mine after 10 years as i didnt like the look of the old one without having any issues but there was a noticeable improvement in driveability after that