Turkington started the day 27th and last on the grid but shot his West Surrey Racing-run BMW 330e M Sport back into contention with storming drives over the first two races.
That put him third on the reversed grid for the finale, and Turkington got himself ahead of the front-row-starting Motorbase Performance Ford Focus ST of Dan Cammish at the start to move up into second.
From fifth on the grid, the Motorbase Ford of championship leader Ash Sutton was squeezed into the pitwall when the gap he was aiming for inside the Excelr8 Motorsport Hyundai i30 N of Tom Ingram vanished.
That damaged the right-front suspension, and the stricken Sutton received a further punt from Josh Cook at Old Hall before parking the Ford on the grass on the inside of the circuit.
That caused a safety car, with poleman Adam Morgan leading Turkington and Jake Hill in a WSR BMW 1-2-3.
Two laps after the restart, Turkington drew to the outside of Morgan at Lodge, then hung on around the outside of Old Hall before audaciously taking the lead when he seized the inside line at Cascades.
Morgan, shuffled onto the outside, also lost out to Hill, who then set fastest lap as he chased down Turkington.
Source: Autosport