In the late 1980's, Timothy Graves, a British researcher, set hilmself the somewhat dauting task of going through the combat reports of the five leading British aces, one by one, and assessing the validity of their claims against a common standard. He then argued that of the 47 victories attributed to Bishop while he was with 60 Squadron, only 5 were independently witnessed - the lowest number of the 5 aces!
In his book "The Making of
Billy Bishop" author Brereton Greenhous, using a somewhat different statitical method finds that 3, perhaps 4, that were independedly witnessed.