I think the police asserted that they had been called at 3:26am, and possibly also suggested that Julie had been called at 3:15, causing Jeremy to give an earlier time for Nevill's call than he would otherwise have done. It seems he hadn't checked the time early on, and so had to go by what the police were telling him.
The problem with revealing Nevill's call was that attention would then shift to the apparent staging of Sheila's body.