thanks RW,
yes the terminals were marked - thanks
this is the alternator regulator - you can see "L" and a bit of the "S" on the other side.
And with the engine running there was no voltage on the "S" terminal of the harness!
Managed to get a thin wire into the plug and run it to the B+ terminal - all fixed, no battery light on the dash.

I'll have to consider a better solution - maybe get another harness plug with the extra wire and re-terminate.

Also since the front of the car was off the ground I turned the steering lock to lock 20-30 times to get bubbles out - seems much smoother.