BBC/ITV are FTA therefore if they can't get a picture then chances are the dish is too small or not aligned correctly and they have no signal. Did the installer mention if they could receive anything off Astra 2D? If the dish is big enough then perhaps they are not selecting the correct BBC or ITV - Some come in stronger than others.
C4 is FTV so if they are getting an upgrade screen then the dish is big enough and the channel should come in soon. If they already get C5,
Sky 3 etc then the problem may be somewhere else. What digibox are they useing? what size is the dish?
If they are getting the signal 'no signal being received' then the channel cannot be received with the present setup.
Cheers
Nick