I’ve discovered following works with Spectrum Lab software on my Dell M6800 laptop:
Plug audio cable into microphone socket on Dell M6800.
Select line in rather than microphone when pop-up box comes up on laptop after plugging in audio cable.
Select Realtec line in on input options on Spectrum Lab.
To construct the VLF aerial, I am still waiting for 22swg enamelled copper cable and 3.5mm audio cable to arrive.
I already have the capacitor required to use on one end of cable to prevent shorting of the audio input on the computer.