As you're in a safe seat, AV wouldn't make any difference. At the last three elections, in the North Durham constituency Kevan Jones polled 67.2%, 64.1%, and most recently, 50.5% of the vote (with the Tories second, and the LibDems third, both on about 21%).

In all three elections, AV wouldn't have progressed to second preferences, so anything beyond your first preference would a waste of effort.