Crankshaft position sensor, as so (old one shown):

I needed most of this lot:

Plus a hacksaw blade to guide the T30 bit into place.

Here’s the offending item. It’s under the plug with two yellow wires. Just visible in what appears to be the top right corner of the plug is the bolt that holds it in place.

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If you should ever have a need to change a crank sensor on a Smart Roadster you have to remove and unplug the throttle body, prise off the hose leading into it, squeeze both hands into a narrow gap so that you can get both index fingers on the plug, squeeze and remove, then assemble a T30 bit, a UJ and an extension and use luck and something long and thin to guide the bit into the bolt. After all that it should hopefully unscrew and you should also hopefully be able to pull the sensor out. If unlucky you may need to drill it out. Reassembly is the reverse of removal but with excellent opportunities to drop the bolt and then spend some time looking for it