Just installed my 360 box and mini box last night.
The short version -- Use the supplied HDMI cables!!
Long version below:
Installation took ages, because when I turned on the box I got the Welcome message followed by the please wait message, and the system updating message, then NO SIGNAL on the TV.....waited nothing.
I run my home network on Ubiquiti Kit so I could see the boxes were online and downloading updates, and I received a text to say an activation signal had been sent but still NO SIGNAL on the TV.
Down powered waited booted up and went through the same process all again....waited. Still left with NO SIGNAL on the TV.
Now, my TV is mounted on the chimney breast with the HDMI lead (3 meters long) fed through the wall, eventually I used the supplied (short) HDMI lead dragged across my wall with the box sat on a table. Huzzah!! I have an image and can finish the setup.