Nailing my colours to the mast here.......
DH and I think the murderer is Petrie, he had the opportunity to hide the hammer etc in the loft at Dessie's house.
Was it the rifle that Dessie had in the bag when the group went to visit the hydroelectric dam? Or was he telling the truth, and it was his curling kit?
My money says Dessie was carrying his curling gear, Petrie had the rifle hidden in readiness, and he killed 'red haired boy' (can't remember his name), attempted to kill Kieran, and then shot Dessie, attempting to make it look like suicide!
Of course, all the above could be utterly, utterly wrong.....I shall find out tomorrow evening!