In the appeal notes, there is mention of another phone call.
"31. Armed officers from the Essex Police Tactical Firearms Unit arrived at the farm at about 5
a.m. There was further conversation with the appellant. At about 5.30 a.m. he said to PS
Adams, “What if anything has happened in there, they are all the family I’ve got”. He
became visibly upset and asked to telephone his girlfriend, Julie Mugford, who was later
driven to Essex from London. "
That doesn't say he actually made a call, just that he asked to make a call.
Around 10pm Bamber called Julie for around 17mins post murders - known as the 'tonight's the night' call.
Called Julie sometime between 03:10 and 03:25 - the 'somethings wrong at the farm' call
Called Julie again at around 05:45 - told her not to go to work, police would pick her up and bring her to Goldhanger.
that second call seems to be the one referred to in the Appeal notes
https://murderpedia.org/male.B/images/bamber_jeremy/20011745.pdf page 6.
How long would it take to drive from her home to White House Farm?