Three-time title winner Sutton, who has switched to a Motorbase Performance-run Ford Focus for this season, stamped in a time with just under 10 minutes remaining in the afternoon to eclipse the morning best of Jake Hill by 0.137 seconds.
It was a qualifying simulation for Sutton, who switched on his headlights in the gloom in case anyone was in the way – they weren’t – and produced a kerb-smothering special to vault to the top.
Quickest in the afternoon to that point, although he hadn’t improved upon Hill’s morning best, was Dan Rowbottom, who ultimately found himself trailing the NAPA Racing-liveried Ford by 0.232s.
“We ran through the test plan, the changes we wanted to make, and they seemed to steer the car in a good direction,” Sutton told Autosport.
“Everyone was chasing tyres by the end of the day [on the high-wear sweeping bends], but it was nice to make improvements in terms of balance.”
Sutton, whose best time was 1m15.607s, said he set that time on new Goodyear tyres, and pointed out that most laps from the low-1m16s down would probably be on fresh rubber.
“That lap was just so we got an inkling of what it’s like at full pace,” he added. “It was kind of as expected, although it wasn’t perfect.”
Hill led the morning session from Tom Ingram by 0.215s, and that performance was good enough for second overall on the day as he gears up for his maiden season in a West Surrey Racing-fielded BMW 330e M Sport.
Rowbottom’s afternoon best was good enough for third on the day and best of the Team Dynamics Honda Civic Type Rs.
The bearded Midlander did most of the team’s running thanks to a bizarre clash in the morning session between his team-mate Gordon Shedden and the WSR BMW of Stephen Jelley.
Both were on out-laps at the time after a red flag, and the damage forced both teams into lengthy repair jobs.
Jelley emerged mid-afternoon, while Shedden finally got out with around 40 minutes remaining, and set a time good enough for fifth overall.
In between the two Hondas was the Excelr8 Motorsport Hyundai i30 N of Ingram, who was in the top five in both sessions with the new bespoke Hyundai engine produced by long-time tin-top power specialist Swindon.
The second of the NAPA/Motorbase Fords, driven by Dan Cammish, wound up in sixth spot, with the BMWs of Adam Morgan (Ciceley Motorsport) and four-time champion Colin Turkington (WSR) next up.
Turkington was on a lengthy race run, setting a string of 1m16s, when Sutton and Shedden slammed in their late improvements.
Dan Lloyd looked good in the second of the Excelr8 Hyundais to go ninth, while veteran two-time champion Jason Plato rounded out the top 10 on the day in his BTC Racing Honda.
Plato was the fastest of the drivers powered by the new M-Sport TOCA customer engine, with his team-mate, Thruxton specialist Josh Cook, in 12th position after an overheating exhaust melted the brake lines.
