Bedford FC await the Royals in the final after they inflicted a second cup defeat on their rivals after knocking them out of the FA Cup in August.
Biggleswade could have taken the lead in the ninth minute but Kane Russell got under his shot and it sailed over.
Potton had the next chance as strong forward play by Aaron Murrell resulted in a shot that was well saved by Robbie Ponting.
Biggleswade’s Hunt and Russell were winning the midfield battle but they struggle to convert this into shots on target.
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However they were awarded a penalty on 29 minutes. Aaron Murrell won the header first but Jordan Wright went to ground and the referee gave the spot kick. David Parkinson seemed to scuff his shot as it hit the post and the resulting follow up from Hunt was well over the bar.
Just four minutes into the second half Potton were awarded a penalty when Nick Bines’ shot was deflected away by the hand of Lee Bilock. Danny Webb made no mistake striking the ball into the far corner.
The game was different now. Dias and Mclean were winning lots in midfield and Webb was sitting deeper so Biggleswade were getting no joy in wide areas either.
With the clock ticking down into the final third, Biggleswade should have equalised. Blatch committed to a challenge which was skipped past and the resulting cross was headed over by David Parkinson.
In injury time Potton doubled their lead. Webb skipped past his marker with ease and passed the ball to Hadley Boughton whose shot went in at the near post via a deflection.