The more we pick this apart, I think between us all. We can come to a more convincing theory. If Lucan made time, did he drive to the Claremont club, in his Mercedes. To give himself an alibi, as I said before. You would not kill you wife yourself and he didn't. If the car was close, cold night. It would have cooled down quickly, if he didn't drive far. Flat battery, in his haste, he left the lights on. But another point, if he was already driving his car, why would he borrow the Corsair. To escape after he killed Sandra and attempting to kill Veronica ?
As for the mail sack, it would not be that hard, for an X service man. As another poster said, he was fit. Mail sacks were everywhere, in the 60's / 70's.
SomethingI did miss in my main theory. Lucan said he saw someone attacking someone in the basement. So, How on earth did Sandra, get into the mail sack. So he saying that, between him seeing someone being attacked and him entering the house. Sandra was put into the sack, that just doesn't make sense. He would have made, a hell of a commotion. That would have made the alleged perpetrator flee. But no, someone then attacks Veronica. That I think, clear up that point. Lucan didn't see an intruder, in the basement, attacking someone. Unless of course, Veronica was setting him up.