More useful comment guys and in particular, Lee, thanks for your input and pictures.
So the Pico is, I now assume, an expensive device that can plot voltages and /or currents over time, presumably using an inductive clamp, so you can leave it and look later?
And what do you mean by 'rolling the lock over'? [When I'm charging I just prop my boot open slightly with a polystyrene block and note that the central locking 'locks up' but doesn't light the door LEDs].
Oh, and by the way, my car's outside so with no cover I need to prepare for any bad weather ...
Julian