Surely can only be the signal. Must be just ok for the rest but not quite ok for Dave. Could check the connections too. Possibly a signal amp for the aerial might help.
Is it fine on all the other channels on the same MUX as dave, which are?
Pick TV
Dave
Really
Dave ja vu
E4+1
Big Deal
Create & Craft
price-drop tv
Gems TV
Challenge
Food Network
Sky News
COMMUNITY
ADULT smileTV3
ADULT PARTY
ADULT Blue
ADULT Babestn2
talkSPORT
Do you have other TVs in the house? If these are OK it points to a dodgy connection of either the scart lead or the aerial, especially if the aerial is on an extension.