Was the 12.4V measured also with the glow plugs on? As that sounds pretty good. I would expect the battery voltage to drop to 10-11V with the glow plugs on, and the voltage at the plugs to be with a few 0.1V of battery voltage.
Ah, that'll be the missing link then! No, I measured before switch on just so I knew whether the battery was healthy (it should have been, it's brand new!).
If you expect it to drop while firing up the plugs, 10.5 perhaps isn't too bad. I'll try again at some point, though not sure when...