What is this?

Under bonnet left side by tool kit. Replaced starter then she wouldn't start, looking under hood in case I bumped some wire off, noticed rubber boot was half off this, when putting it on I pushed the top, it clicked, the fired first time. Maybe an overload of some sort? Freelander 1, 2000.

That's the

Inertia Fuel Shut Off Switch​

cut out switch if you have accident, in some cars it is inside the car. IE the Rover 75.

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