Hosts Belton made a competivive 196-7 thanks to 53 from Daniel Turner and 50 from Tim Jack, but it was still an achievable target for a Boston side that has been in great form.
However the league leaders had an off day with the bat and were skittled for just 104.
Belton's win opened the door for Grantham to take over at the top if they could secure victory, which they duly did with a 80 run win over Market Deeping II.
Grantham posted an impressive 230-3 with 61 from Jonathan Birkin and 51 from Andrew Mudie.
Deeping withstood the Grantham attack for long periods to ensure they kept wickets intact, but they never looked like reaching the target and finished on 150-8.
Billingborough kept up their challenge with a six wicket win over Woodhall Spa, reaching their 160 target for the loss of just four wickets, while Bourne continued their climb up the table in the closest match of the day against Sleaford.
Bourne reached 175-8 despite a five wicket haul for Richard Flower.
Martyn Miles' 76 saw Sleaford closing in on the total but the visitors surrendered their final wicket just six runs short in the last over of their innings.