I lost BBC4 HD a long time ago (along with CH4_7 HD). When I last asked about this I was told that it was likely that my signal strength was too good! I tried turning off the signal amplifier*, which dropped signal strength from 100% to about 75% on the likes of BBC1 HD, then retuning but I couldn't get the channels back. I've not tried fitting an inline attenuator to see if that makes a difference - I don't really watch BBC4 or 4_7 very much so I've not had the incentive to do so.
* A peculiarity of my setup ... the TV is in another part of the room from where the aerial point lands, so I have some thin coax cable running through a channel in my floor (below the laminate) to another coax connector near the TV. As the cable is thin it is higher resistance, so I needed a signal amplifier when I had a VCR connected which looped through to the TV. I've just kept that setup since I've had the BT Youview box.
Thanks Ian http://www.siteguru.co.uk/be/ADSL2PlusForDummies.pdf - Having ADSL issues? Then read this PDF Please don't click the star beside my name - too many stars makes you look like a swot!