I remember this.
Julie had already betrayed Bamber 20 days after the massacre by telling another person what she knew. Dispite this Bamber still apparently jilted her so he could go to St Tropez with Brett.
After going to the police, a jilted Julie was supposed to somehow get back into Bamber's life & get an audio confession from him.
The police didn't attempt this. History has shown they didn't need to.