The two had been tangled in an early battle for victory, but just after Buchan hit the front, the McAMS Yamaha rider skittled into the dirt after losing the rear and crashing out.
But it was drama before the start for Santander Salt TAG man Dan Linfoot, who pulled into pit lane to swap his wet tyres for an intermediate front and slick rear, meaning he would have to start at the back of the grid,.
As the lights went out, it was Mackenzie who eclipsed pole sitter Fores with a lightning launch as Buchan slotted into third ahead of Tyco BMW’s lone rider Christian Iddon.
While Jason O’Halloran and Andrew Irwin tagged onto the back of the leading quartet, Tommy Bridewell led the chase from seventh on his Oxford Ducati - over two seconds back on the front-running bunch.
Buchan slipped ahead of Fores at the bottom of McIntyres to take over second on lap five and soon tagged onto Mackenzie’s wheel as he and the home hero stretched a gap on Fores and Iddon.
Mackenzie had a moment at the exit of the Chicane on lap 10 before he and Buchan made contact coming out of Clarke’s and while the Yamaha rider held onto the lead, Buchan stole the dry line away into the Hairpin to hit the front.
Back in fifth, O’Halloran’s Yamaha coughed smoke and the Aussie was subsequently black flagged, but it was all over for the McAMS duo moments later as Mackenzie lost the rear at Clarke’s and put paid to his hopes of victory.
The Scotsman’s high side left Buchan two seconds up on Fores and the former National Superstock 1000 champ continued to inch away on the Spaniard.
While Fores and Iddon continued to battle for second, Scott Redding was surging through the pack as he stuck a pass on Bridewell to move into fourth place, but the Be Wiser Ducati rider still had three seconds to make up on third-placed Iddon.
But Redding made the best of lapped traffic and with six laps to go, he sliced Iddon’s advantage down to just over a second.
With just over four laps to go, the former GP rider slid up the inside of Iddon at McIntyres and, with the fastest lap of the race in hand, he went straight after Fores at the exact same place on the very next lap.
Eight seconds up the road and with six seconds to hand, Buchan powered across the line for his very first BSB victory.
Redding’s heroic charge from 16th on the grid to second place ahead of Fores and Iddon puts him within five points of championship leader Bridewell, who came home in fifth.
Buildbase Suzuki’s Bradley Ray bagged sixth with OMG Suzuki man Luke Mossey and the second Be Wiser bike of Josh Brookes taking seventh and eighth.
Luke Stapleford and the lone Quattro Plant JG Speedfit Kawasaki of Glenn Irwin completed the top 10.
