At the halfway stage of the season, the top three teams have opened up a 19-point gap over the chasing pack.
Bracebridge Heath remain in first place – going for their third successive championship and their 11th overall – with a 16-point lead over Grantham in second who have a six-point advantage over third-placed Bourne.
Woodhall Spa stay fourth, despite losing a tight game to Grantham with Lindum fifth.
But below them just six points separate the next five teams – Alford leading the pack with Grimsby and Market Deeping just above the relegation zone.
Louth’s defeat by Lindum puts them back into danger in 11th place just eight points up on bottom-placed Spalding.
Tight bowling from Bourne restricted Sleaford to 171-9 with Jack Berry taking four wickets before going out to open and getting his side off to a good start with 32.
Fellow opener Jordan Temple went on to make 46 and with three wickets down, Pete Morgan and Carl Wilson joined forces to see Bourne to a seven-wicket win.
Fixtures (Saturday, noon start): Alford v Louth, Boston v Sleaford, Bourne v Bracebridge Heath, Grimsby v Spalding, Lindum v Grantham, Woodhall Spa v Market Deeping.