Did washing machines in 1985 have a quick cycle option?
If so they could be washed while he was at Goldhanger. Between around 3am & 3.36am.
It doesn't always remove stains, though, especially blood. I wonder if he factored in not getting blood on his person: twins shot as they slept, no resistance, parents shot in bed from a distance, Sheila led by the hand like a lamb to the slaughter. The only slip was Nevill: how did Jeremy think his father would react? We are told "
the old man put up a good fight for his age", no feeling, no remorse, as he related these details to Julie. He may not have worn overalls after all.