Grayson grabs a hat-trick as Old Northamptonians claim glory at Twickenham
and live on Freeview channel 276
Luke Bond, Jake Diggin and Rory Seymour were also on the scoresheet as ONs recovered from 14 points down at half-time to see off Horsham 40-35 in the showpiece.
Horsham scored five tries of their own, through Aaron Linfield, Rich Tredgett, Jordan Smith, Ed Mannhardt and Jonny Gowar, but were edged out in a classic at Twickenham after an unforgettable second half from ONs.
Horsham had come out the stronger of the teams, with scrum-half Linfield crossing after just a few minutes.
ONs did hit back quickly with a score of their own from No.8 Bond, who powered over to level the scores after 10 minutes.
Horsham then found a purple patch in the second quarter of the match, dotting down twice, first through Tredgett and then flanker Smith as they pulled into a 21-7 half-time lead.
But ONs were not going to lie down and roll over for their opponents, and they had a quality 25 minutes to go from two scores down to take the lead. Diggin crept over the line to respond quickly in the second half, before two excellent Grayson finishes in the same left-hand corner suddenly put them into a 26-21 lead.
The next score was crucial and it was ONs who got it once again, this time from full-back Seymour, and they led by 12 points with just 10 minutes left on the clock.
Horsham scored another try to give themselves a chance when Mannhardt went over, but Grayson scored his third to seal the tie, before Horsham grabbed a consolation score from Jonny Gowar.
Old Northamptonians: Rory Seymour; James Greensmith, Ben Rivans, Mikey Bellamy, Joel Grayson; Joe Heslop (c), Lewis Barker; Jake Diggin, Jordan Brew, James de la Cruz; Toby Wallace, Samuel Howes; Harry Stone, Lochlan Brown, Luke Bond.
Replacements: Owen Postlethwaite, Dan Hughes, Matt Dryden, Brandon Tacticos, Sam Connor, Seb Henry, Sam Page.