Hmm same problem here with the UM7400.
Apparantly it is because your ariel signal is far too strong and this will cause interference with the tv, therefore stopping some services/channels/HD channels. LG tv's need between 70-85 signal strength.
I was also told it was freeview at fault.
However, my older LG tv (last year's model) runs absolutely fine on vision tv, planet knowledge, channel 261 etc.