Both Dave SD channels are on 12129 MHZ, Vertical. Try another transponder by perhaps using the orbital service list instead of just the bouquet created for your region.
Free versions of channel 4 HD are on:
12363 MHz Vertical
12168 MHz Vertical
12226 MHz Vertical
11126 MHz Vertical
If a manual enigma2 scan, single satellite, of 28.2 gets less than 800 channels, you have a problem that you need to investigate. Might be LNB failure as you suspect.