With the Extra Preliminary Round replay tied at 1-1 Potton keeper Will Cook was shown a red card for bringing Biggleswade forward Ioannis Varouxakis down outside the penalty box.
Substitute goalkeeper David Salmon, nearly 40 years of age, made two sensational saves before Courtney Boughton scored the winning goal in the second minute of added time.
The replay at Second Meadow, forced after Friday’s tie ended goalless, was played at a fast pace. Danny Webb played the ball to Calum Forster whose low shot from 30 yards went just wide of the post.
A Potton corner was headed back by James Meredith to Aiden McClean who saw his effort stopped on the line. A mistake by Calum Forster let in Varouxakis whose effort rattled the cross bar and went to safety.
Nick Bines had a good run and saw Aiden McClean’s effort go just over the bar.
The deadline was finally broken after a good passing movement between McClean and Webb saw the ball go to Meredith who fired home from 20 yards to give Potton the lead.
Webb, who was proving fast and dangerous on the right side saw his good run cleared for a corner from which he headed wide.
In the second half Biggleswade came more into the match and Varouyakis saw his effort saved by Cook.
And with 20 minutes remaining they were level. A good through ball found Charlie Black, his low shot was pushed away by Cook but Black followed up to equalise.
Howard-Dobson then showed a moment of brilliance before driving a shot narrowly over the Potton bar.
With the game entering the final minutes the drama unfolded. Potton’s James Sage was caught in possession on the halfway line. Biggleswade played a quick through ball to Varouxakis who was clear on goal but Cook brought him down outside the penalty area and was dismissed. Gareth Hunt took the free kick and saw Salmon leap to push the ball onto the bar.
Potton were up against it with ten men. Good defending by Forster stopped Varouxakis, before he had a close range header well saved by Salmon.
But from there the ball was quickly played up to the other end where Murrell was clear through on goal. Gowans blocked his effort but it ran to Courtney Boughton who scored just inside the post to seal tbe win.
Victory sees Potton bank £1,500 in prize money and sets up a Preliminary Round tie at home to Southern League East side AFC Dunstable.
Potton host Bourne Town in their first UCL Division One match on Saturday.
Biggleswade United launch their SSML Premier campaign at home to Holmer Green on Saturday and travel to neighbours Stotfold on Tuesday.