He had the foresight to get a blank tape then. I never had a spare tape.
If he planned it, as is the prosecution's case, then he was also rather fortunate in the sense that police didn't initially have reason to doubt him, and allowed vital evidence and opportunity to slip through their hands.
Had Nevill managed to inflict a wound, it might all have fallen apart rapidly.
It seems to be the general consensus that Nevill did battle with his assailant, but it is not necessarily so. He may well have just taken a beating and the kitchen upturned to suggest a frenzied battle. Neither Sheila nor Jeremy showed any significant signs of a fight, and Sheila demonstrably so.