My view is he phoned his AM from WHF around 3am.
This gives him an easy 26/36 minutes to get back to WHF & phone Chelmsford police. Either before or after washing & changing.
Ringing at 3.10am it is still possible but more of a rush.
Bamber would have said 3.10am either in error, or he didn't want the time gap before he called Chelmsford police to be too long.
If BT could trace the call & said it was at 3.00am, Bamber can just say 'I was half asleep & got the time 10 minutes wrong'.