Actually, like with everything else, with time explanation changed within each interview, but the oldest and in my opinion closer to the "truth" interview in which the matter is discussed is this one:
http://www.fleetwoodmac.net/penguin/archive/tijd.htm
P.S. Now since The Blue Letter Archive doesn't work properly I can't find the one in which they talked a bit more about the McVies, I actually had trouble finding that specific interview above too, however it was mostly due to John's alcohol addiction.