I would think that the HDMI issue is clouding the waters here.
You should be able to watch the TV via IT'S OWN BUILT IN TUNER, even if the aerial is fed via the Humax. As long as the Humax is plugged into the mains, and the power switch at the back is on, I.E. you can see the display (clock) on the Humax, then that should work. One caveat is that the power saving mode on the Humax should be OFF, so the Humax tuner is always on.
Is it possible to test the Humax on another TV?
Forgetting the HDMI connection, do you have a SCART lead you can use instead?
Having said that, SCART/HDMI are independent to the RF anyway so even without either connected you should be able to watch TV via it's own tuner. Just thinking out loud here now, ignore my rambling!!
I don't know what else to suggest.