It seems conclusive that Bamber said this to Julie in his 10pm call to her -
He had rang her at 10pm. So wanted to tell her something.
Julie told SB she may be hearing from him later.
Julie did hear from him later.
Bamber has not denied saying this.
----------
'Later' would mean later that night. If he meant tomorrow he would say 'tommorow'.
The 'may be' was on the provision he went ahead with the massacre and was successful.