The correct group aerial for Dover is Group C/D. According to the
Digital TV Group website the reception channels are Mux 1 (BBC) 68, Mux 2 (ITV) 61, Mux A (CH5) 55, Mux B (BBC 4 etc) 58, Mux C (Sky 3) 57, Mux D (Virgin 1) 60.
Although channels 42 & 45 are used from Dover, DTVG do not mention these for normal reception. One likley problem you may have is interference from the continent. With the old analogue you knew when you had co-channel interference (the old "venetian blind" effect) on the picture. Unfortunately digital doesn't give you these clues.