If it was Sheila, everyone must have died between 3.20am - 3.48am.
Neville was still alive and phoned Jeremy at 3.20am saying 'Sheila has gone crazy and she's got the gun'. Surely Neville had not already let Sheila kill everyone else and reload. Had he ?
The police arrived at 3.48am. They do not recall hearing any shots or seeing anyone going 'crazy' inside WHF. Apart from the 'trick of the light'.
Sheila certainly killed Neville after 3.20am. Beating and shooting him. If the others were still alive, she then shot them. Then shot herself, once. Took the silencer off. Put it neatly away. Showered and changed. Painted her toe nails. Then shot herself again. All in less than half an hour.