If I knew the layout of the property I would head for the nearest door.
The police, relatives & juries view is that there were no phone calls from the house.
Jeremy crept upstairs & started shooting with a silencer/moderator attached. Everything had gone to plan with June, who was shot without resistance. However Neville woke up & got downstairs into the kitchen dispite being shot, either to escape or phone the police. He was not surprisingly caught up & beaten, perhaps because the gun was empty. Then fatally shot in the head with a reloaded gun.
Jeremy then returned upstairs & led a half asleep Sheila into the main bedroom & shot her. He panicked after realising she was still alive after the first shot & fired again.
When the twins were shot is neither here or there. Whether shot first or last, they did not wake up that night.