There's so much conflicting evidence:
"I am aware that Sheila,her two sons and her parents have been found dead in Essex, and that it is suggested that Sheila killed her family and committed suicide. In hindsight I believe that Sheila would have relapsed into a state of acute psychosis, probably having a firmly held belief or delusion involving concepts of good and evil, and certainly paranoid,possibly involving her mother. Sheila is likely to have been in a disturbed psychotic state at the time of the tragedy, although in my experience with her there has never been any issue of threats or violence towards her family." Statement Dr Hugh Cameron Ferguson 08/08/1985
"I would like to add that if Sheila had been over-sedated she would have been vacant, difficult to converse with, and it is possible she would have slept quite soundly and deeply but this is not absolutely certain. I would say that her whole physical performance would be reduced."
Statement Dr Hugh Cameron Ferguson 18/09/1985
This second statement does seem to tally with Sheila's behaviour that final week: her demeanour at Colin's party and during the ride down to the Farm, her lackadaisical attitude and vacant stare at the shop in Tiptree and round the kitchen table that final evening.