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[Dieselman]

You could pull out the fuses, or unplug the Hydractive Ecu to force hard mode to be selected, which would be one test you need to perform anyway as part of the diagnostic process.
You can also disconnect each electrovalve return pipe and observe the flow of fluid...which should be a spurt as the valve changes state. Continuous flow indicates a leakage issue.
If you can't go under the car try disconnecting the return at the reservoir.

I don't have the fuse numbers, or return pipe info, to hand.