Smiths Racing Triumph’s Mossey topped the timesheets with his 1:45.444 effort, with David Paton second fastest.
There was also drama as championship leader Jack Kennedy crashed out, and he will have to start from the back of the grid after what was a disastrous session.
Christian Iddon, riding for Oxford TAG Triumph, was third quickest with Ben Wilson fourth and Luke Jones fifth.
1. Luke Mossey (Smiths (Gloucester) Racing Triumph) 1:45.444
2. David Paton (Double M Yamaha) +0.551s
3. Christian Iddon (Oxford TAG Triumph) +0.634ss
4. Ben Wilson (Gearlink Kawasaki) +1.893s
5. Luke Jones (777RR Triumph) + 2.425s