York City had a chance to regain top spot with King's Lynn out of action - but they slipped to a second defeat of the season at the hands of Darlington.
The Minstermen's defeat opened the door for the chasing pack - and they took full advantage.
Bradley Bauress, David Morgan and Jack Sampson had Southport looking comfortable against Blyth Spartans.
Rhys Evans got one back for the hosts but Ryan Astles made it safe for the Port just before the hour mark.
Brackley Town are just behind after goals from Matt Lowe, Thierry Audel and Lee Ndlovu saw them comfortable winners against Curzon Ashton, 3-0 it finished.
Luke Parkin scored the pick of the goals for Farsley Celtic as the Celts edged past Leamington.
Rob Ramshaw had Spennymoor Town in front before a Mark Anderson brace after the break helped the Moors past Bradford (Park Avenue) - Carl Magnay saw red late on.
Chester could only draw with AFC Telford while it was also a goalless draw between Hereford and Guiseley.
Kettering's resurgence under Paul Cox continued with a fine win away at Alfreton Town - Tre Mitford's brace enough for the Poppies.
Elsewhere, Jake Moult's strike for Altrincham cancelled out a Kacy Butterfield opener for Kidderminster Harriers as they shared the spoils.
Saturday's National League North final scores:
Alfreton Town 0-2 Kidderminster Harriers
Altrincham 1-1 Kidderminster Harriers
Blyth Spartans 1-4 Southport
Boston United P-P Gateshead
Brackley Town 3-0 Curzon Ashton
Chester 0-0 AFC Telford United
Farsley Celtic 2-1 Leamington
Gloucester City P-P King's Lynn Town
Hereford 0-0 Guiseley
Spennymoor Town 3-0 Bradford (Park Avenue)
York City 0-1 Darlington