Checked the fuses and all seems okay... bugger, I was hoping for something simple!
As for checking the vacuum hoses and solenoid, I'm not really sure where / what I should be looking at.
I can tell you though, that when I turn the ignition to the ON position (not starting it), the 4L lights come on, then go off, then come back on, I hear two clicks (bit of a hiss) from the front right... dunno whether this is the solenoid? Never noticed it before this.
Starting the engine, the lights stay on, and thus far I cannot hear any change when I turn the knob from 2H to 4H... (leaning out of car as far as arm can reach, maybe should try adjusting knob with toes!)
Hopefully soon I can get a trusty assistant to change while I crawl around underneath listening for something... anything
Any other tips welcome, thanks guys.