The incident takes place in the 82nd minute. Dartford in white and
Maidstone in Amber. Dartford player shoots from edge of area, attacker
in an offside position ducks to allow ball into goal. GK approx 3 yards
to left of offside player dives but cannot reach the ball.
His direct line of vision is not influenced by the offside striker, unless he sees him out of the corner of his eye.
I'm tempted to agree with the referee here, but I'm not sure why.
Assistant flags for offside but is over-ruled by ref.
That said, I've looked at the Premier League's advice to referees now and it would appear to indicate that the act of ducking to get out of the way of the ball is enough to count as interference.
I've posted it on a refereeing website where the consensus so far is that it's a marginal decision and it would probably have been best NOT to over-rule your assistant.
Still, looked like Dartford had some excellent chances anyway.