Returning to Schrage Musik - Eyes of the Night is a book describing the air defence of NW England 1940-43 which consisting in the main of non-radar equipped Defiants. These normally operated on moonlit nights detecting enemy aircraft by sight.
The book records 23 combats with Ju88s and He111s resulting in 12 being destroyed. These bombers had ventral gun positions and the Defiants usually attacked from below at distances between 25 to 150 yards. But only on two definite occasions did the bomber fire back and in both cases the Defiant was making a repeat attack. So one is tempted to think that even if the Lancaster had a ventral turret it might have made little difference.