20 minutes isn't really a great deal of time when you consider the carnage in the farmhouse. How did he manage to successfully guess that the police wouldn't suspect him from the off - given that he had left Sheila with two gunshot wounds to the throat after having set her up as a suicide victim?
How did he know that the police wouldn't forensically check his person or cottage for traces of blood or excessive washing? He apparently knew that he had completely botched Sheila's 'suicide' when he was washing himself!
I could turn that around and ask why there was a twenty-minute delay in telephoning Police, which is explained, of course, if there were no telephone call from Nevill in the first place. His attitude to Police had been conditioned by his father, who labelled the local constabulary "
Dad's Army", and his recent brush with Police when he crashed his Vauxhall Astra only weeks before confirmed this interpretation that they were impressed by the wealth and status which Jeremy's association with White House Farm afforded.