I can jump my 1981 300SD with two screwdrivers. There is a plastic box under the hood, right side (in USA) with four screws holding wires. The box has a cap on it and measures about 2 inches by 4 inches. Located just in front and below the antifreeze reservoir. Connect the screw closest to the dash (call it #1) and screw #3 using crossed screwdrivers. If the starter is OK, it will spin just like using the key. (Of course key must be in the on position.)

This is the SD but I would be surprised if the 300D does not have it too.

Good luck.

Finally found a pic of the jump box. It might be on the other side in UK.

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