Bamber was never going to call the police himself & pretend to be Nevill. That is much too elaborate and risky.
He also wanted to tell the police that Nevill wanted to keep family matters private.
He gambled on this, and his remark to Liz Rimington that
"you can convince anyone of anything." summarizes his position. The stage was set for the unfolding tragic drama, Jeremy in the role of victim as five members of his family lay dead yards from where he stood, all having met their maker at Jeremy's own hand.