In those circumstances an effective alibi would have been her spending the night with him in Bourtree cottage 6.8.85
And after a little digging that may have invited more scrutiny if I'm honest.
That is what I meant.
The advantage here is Julie would be less likely to break. Even after they split.
Being an alibi in the days & weeks afterwards is a lot more serious.
If she did break, Jeremy could just deny it. As he has done for 41 years.