@andrewwynn clearly you are an electrical whizz..alas dont have a fluke like you but can try capture on phone camera what i see and watch frame by frame...excuse my ignorance but where in lighter socket do i touch multimeter probe to do your below step:

If you take the voltage drop measured from the cig. lighter socket to the B+ jump point under the hood and multiply by 285 you should arrive at starter current.

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N