Stox star Paul targets final date
Harrison could only finish seventh at Northampton as Luke Davidson roared to his first major crown.
Mark Gilbank was not in action because of other commitments.
Harrison will now look for better results this weekend in the first World championship semi-final at Sheffield on Sunday (1pm).
He takes his place on the front row in the 20-lap race, aiming to qualify for the final at Coventry on September 4.