The evidence is consistent with Jeremy being involved so there is no question of his guilt. He could very well have involved a third party to do the dirty deed but there is no evidence to support it. That said however, there is every possibility that a third party knew of the plan ahead of time and could also have faced a conspiracy to murder charge.
The evidence to support it includes
1)Jeremys statement to Julie,
2)a police officer thinking they may have seen movement in a window,
3)a police officer thinking they saw a rifle in the window that was moved by someone other than Jeremy,
4) a possible sighting of a hunched up man leaving the scene,
5) lack of marks on Jeremy,
6) no violence at all in Jeremys past,
7)some people say he did not have it in him to shoot people personally,

no one saw Jeremy enter or leave his cottage at the relevant times
9) a possible sighting of a number of vehicles parked outside his cottage on the evening of the murders
10) He certainly knew some very dodgy people in more than one country
any other possible evidence to add to the list any one ?
When you say "Third party" do you mean Julie ?
If not could you name who else you suspect of possibly having pre knowledge of the crime ?
I hope you agree this is worth thinking about very seriously