But, my money is on (3) - Ralph Neville, the south African...
He had fled South Africa amid rumours that he was wanted in connection with a number of shootings, involving white farmers and their families! According to Jeremy he was a firearms dealer! On the morning of the tragedy he fled the UK as a hitchhiker, leaving his mobile home parked up at the farmhouse! It's too much of a coincidence that he was wanted in South Africa for shooting dead farmers and their families, and that he should be working at whf at the time of the tragedy, and he flees the country telling lies to the driver who was kind enough to give him a lift, saying some reporter or other had told him about a family who had all been shot, but Ralph Neville said, he didn't know where the shootings had been! Of course he knew, he knew because the family that got shot were the family that he had been staying with!
I think Ralph Neville was the scruffy looking hunched man seen walking away from the farmhouse after he had helped Sheila to shoot and kill the other four victims!
It is rather astonishing that when he was picked up as a hitchhiker on the morning of the tragedy, that he was standing a 100 yards or so leaning against a wall close to Head Street, Goldhanger, where Jeremy was residing! Maybe he had gone there intending to kill Jeremy, only to find that he was not home at that stage, because Jeremy was still at the scene with the police until about 9.30am...