I have just had a thought, it bugs me how Nevill would get passed Jeremy with a loaded weapon (which is why one of my considerations is that Nevill may have slept in Sheila's room but if not ......) - Jeremy may have ran shot of ammo upstairs, this would give Nevill the chance to rush him. So perhaps Nevill chased Jeremy downstairs? Once in the kitchen it would be a fight to get to the bullets so this may be how and when Nevill was bludgeoned. Once Jeremy got control, reloaded enough to end his fathers life after which, he reloads again and returns upstairs.