I think Caroline's thesis was that Jeremy stayed the night quite openly at White House Farm.
That does not cohere with Len's evidence, and that of his wife.
First, Len says Jeremy told him he was leaving for home after a long day and Nevill had agreed to take the last trailer instead, as long as Jeremy left it in the field.
Next, Dorothy Foakes hears Jeremy drive off (she doesn't actually see him drive off, but the sound is similar to what she is used to and she assumes it is him).
Then Len sees Nevill do exactly what Jeremy said Nevill would do. Why does that happen if Jeremy is still there? If Jeremy is staying the night, then it was his job to take in the last trailer, not Nevill's.
Later, Len sees Nevill still out on his tractor at 10.30 p.m.
Of course, Jeremy may have arranged to stay the night anyway and lied to Len about why he was leaving early, instead intending to return. But wouldn't there be a risk of that arrangement leaking? Remember that there was a call between Pamela, June and Sheila at about 10.05 p.m., and Nevill could have mentioned to anybody that Jeremy was coming back.
If Jeremy is the killer, he doesn't want to arrange to stay the night, instead he needs to be seen to return to Bourtree Cottage and he needs people to think that is what he has done.