The MJSL team cranked up the pressure, making their dominance count through Moses Seitler.
South were rattled and created few chances, the best coming from the left foot of Toby Levy, with keeper Jordan Warrents saving brilliantly to his left.
Then it was two, with Gabe Silva' bullet header doubling the MJSL's advantage, with half an hour to go.
Tom Cantor then gave South a lifeline, nodding home a corner from Levy.
Lavin equalised before slotting home from the spot to complete the comeback.
South manager Neil Collins said: "it was a great game of football, showcasing the north well.
"We were lucky today and didn't deserve to progress, but the boys battled and dug deep.
"It's back to London for us next week, for a change, and we look forward to representing the North in the semis."
