East Grinstead III missed the chance to take a step nearer to the title after suffering a first defeat of the season at home to Balcombe.
The visitors set 193-9 declared with Ed Murrills scoring 66 and James Taylor 55 and that proved more than enough as the leaders slumped to 96 all out in reply with Gary Paddon claiming 5-43.
That result enabled second-placed Staplefield to close the gap to 17 points with two matches go to.
Steve Barker (46) and Charlie Cooper (31) led Staplefield past Preston Nomads III's 148 all out with just two wickets to spare as James Silk (7-67) caused problems.
Third-placed Horsted Keynes still have a mathematical chance of winning the league but the 40-point gap on East Grinstead looks far too much.
Stewart Fuller hit 100 as Horsted Keynes set 200-8 declared and they bowled out Dormansland for 173 with six balls remaining, Tom Robinson taking 5-48.
Andy Hill took 8-47 as Edenbridge beat Streat by 96 runs while Simon Hardy took 6-49 as Cuckfield III defeated Poynings by seven runs.
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