Harriers, cheered on by a vocal travelling support, gave away a free-kick 25 yards out on three minutes after a push by big centre-back Sall. Good defensive work from Craig Hinton early on
But Hinton reacted well in the box to head away the set piece.
Orient won a corner on six minutes when Martin's attempted left-wing cross was blocked by full-back Clarkson.
Hinton was again there in the air to clear the flag-kick.
Harriers were working hard to frustrate their opponents although Harris caused Brock some problems with a vicious low 35-yard drive.
The Harriers keeper did well to claim the ball at his left-hand post.
Brock was caught dozing on 15 minutes when he was slow out of his goal and McLean nearly capitalised on Sall's headed backpass.
Brock then held a good header from Minton moments later.
But the visitors took the lead on 20 minutes with a superbly worked goal.
Williams won the ball in midfield and fed it to Bennett who played a quick one-two with Larkin before breaking into the box to fire home past Barrett.
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