This requires that your Sky+HD box has the new EPG version.
Press "Services" on the remote control (or from the Guide go to "Options" on the top menu).
Press the down arrow, and then scroll across to "Add Channels".
Press "Select" or the down arrow, to highlight the frequency.
Using the number pad on the remote, enter the following frequency: 11.427
Press the down arrow, and use the left or right arrow to change the Polarisation from V to H
Check that the Symbol Rate is 27.5
and that the FEC is 2/3
, and adjust if necessary.
Press the yellow button to Find Channels.
Once it has finished scanning, find & highlight channel "10510
" and press the yellow button to store it, and then press Select to continue.
You can now view ITV HD by going to "Other Channels" within Services/Options (it's next to "Add Channels") & pressing Select to reveal channel 10510 (ITV HD) & then pressing Select again to view it.
[As ITV HD is not in the actual EPG, the Guide & the Search/Scan banner will not show any schedule information for it. Also, I believe that you cannot use any of the Sky+ features on it (scheduled recordings, Live Pause, Instant Rewind, etc.) - although I think manual recordings should work.]
The ITV HD schedule is currently rather poor, but does apparently show some sort of football.