Not thorough enough to know that Bamber did not live there? Remember Nevilles words from Bamber your sister/ Sheila has got the gun or to that effect, what assassin who is top draw material would leave himself wide open for Bamber to phone the police?
They would have known that.
They would have had very little chance to get all the family in one place at one time.
Maybe their best opportunity was that night.
Jeremy not being there was a minor inconvenience.
They would have forced Neville to make the call.
They would know it was 50/50 whether Jeremy would arrive alone or with Police.
They got unlucky and when they saw the police simply left the farm under cover of darkness.
They could have dealt with Jeremy later - he wasnt the main target.
Then a member of the family got talking with Julie and they jointly convinced themselves Jeremy did it.
They were not prepared to let him get away with it.
So they arranged to plant the silencer and for Julie to confess.
DS Jones had not liked Jeremy from the start and suspected him.
He would therefore be all to willing to uncritically accept Julies confession ...