Runners-up Deeping Rangers crashed to defeat from a last-minute blast by Jason Turner against one of his former clubs.
Saturday’s contest at the Waterworks Field was not as good as the first meeting earlier in the season.
But each side tried to gain the advantage – Rangers’ Scott Mooney winning the first tussle with Leo Adams but curling his shot wide of the far post.
Ar Tarn’s Turner spun in the box and shot, the ball fell for Jake Bettles and Tom Smith’s good block saved Deeping.
Desborough Town made the breakthrough on 12 minutes. Aidan Bradshaw won the ball easily from Dan Schiavi and fed Turner in the box – his shot brought a fine low save from Richard Stainsby but the parry was smashed home by Bradshaw.
Chances came for both sides – Mooney found space to shoot, bringing a flying save from Chris Jones to tip away, while at the other end, Turner curled an effort wide of the post.
Tom Waumsley chased down a long ball and fed Smith, who in turn played Scott Coupland free. His cross went through the face of the home goal without a touch being applied.
Mooney saw his shot deflected into the hands of Jones and, as the half-time break approached, Coupland was played into the penalty box where his effort was palmed away.
Rangers continued their attacking intent as the game progressed.
Mooney wriggled free to set up Schiavi, who skied his effort over the bar.
The equaliser came on 59 minutes. Mooney was fouled 25 yards out, Schiavi’s free-kick was spilled by Jones and Mooney was on hand to pounce and stab the ball into the net.
Ar Tarn responded when Dan White’s long ball freed John Dean, who cut inside and fired goalwards, only to see Stainsby’s full-length save thwart the danger and Bettles saw a snap shot go high over the bar.
Smith sent in a cross that bounced up for Coupland, but his shot was blocked by Aaron Davies.
Mooney then fed Schiavi, whose effort hit Jones who collected at the second attempt.
With time almost up, Mooney got free again and progressed along the byeline.
His shot was taken by Jones, who quickly sent the ball down the pitch, Turner saw the ball bounce up and from outside of the corner of the box swung his foot to sail the ball over Stainsby and inside the far post to get the decisive goal.
Deeping: Stainsby, Marsden, Smith, Burton-Jones, Hunnings, Clay, Waumsley, Coulson, Mooney, Coupland, Schiavi. Subs not used: D Zalejski, N Zalejski, Ramsden.
Deeping face Harrowby United on Tuesday night in their final home game of the season.
They complete the league programme at Boston Town on Saturday.
Rangers meet champions Peterborough Sports in the Hinchingbrooke Cup final at Yaxley on Tuesday next week.