Eighteen runs are scored off the final over for thrilling victory
Bourne booked their place at the Winkworth Cup finals day after a dramatic success in a bonanza of Bank Holiday action at the Abbey Lawn on Monday.
Having comfortably beaten Boston by 69 runs in the first match of the day, Bourne then saw off Sleaford in a gripping finish after scoring 18 runs off the last over to join Bracebridge Heath, Lindum and Woodhall Spa in the final on Monday, May 27.
The Sleaford success helped gained revenge after a narrow two-run defeat on the opening day of the Lincolnshire Premier League campaign.
Sleaford had batted first that day and they again took to the crease first on Monday as they looked to back up their 16-run victory over Boston.
Tom Shorthouse - who had scored a match-winning 113 in the league match at the Abbey Lawn - began brightly with Chamara Kapugedara as the duo put on 65 runs for the first wicket.
Matt Kidd made the first breakthrough to have Shorthouse caught by Dan Bandaranaike for 16, but Kapugedara and Andy Hibberd upped the run-rate.
The Sri Lankan took just 25 balls to reach his half-century before he was caught by Rob Dunn off Bandaranaike shortly after.
Ben Stroud then had Charlie Roberts caught by Pete Morgan for seven before Bandaranaike bowled Hibberd after he had made 23 to leave Sleaford on 109-4.
This brought Ollie Burford and Matthew Wing together and they finished unbeaten on 43 and 20 respectively at the close after putting on 67 runs which helped Sleaford post 176-4 from their 20 overs.
Bandaranaike was the pick of the Bourne bowlers with 2-21 from his allotted four overs.
Bourne also got their reply off to a solid start as Morgan (31) and Courtney Kruger (23) put on 54 runs for the first wicket.
They fell within six runs of each other, but Sam Evison (42) and Steven Crossley (33) soon took over the mantle.
They added a further 59 runs for the third wicket with Dunn then making 28 to leave Bourne needing 18 runs off the final over.
Dunn smashed the first two deliveries of Scott Tite's last over to the boundary, which included four no balls, but was then caught off the third ball.
This left Kidd on strike and he hit a boundary before running two twos to tie the match going into the last ball.
He managed to run a single with Jordan Temple which booked Bourne's place in the finals day at Sleaford later this month after a five-wicket success.
Bourne had earlier successfully defended their 175-6 target which they had set Boston after 20 overs.
Morgan cracked five fours and three sixes in his 51 off just 34 runs while Evison (35) and Crossley (33) also put on solid shows.
Adeesha Thilanchana was the mainstay of the Boston reply as he thumped 12 fours and a six in his 73 which came off 50 balls.
But none of the other Boston batsmen were able to support the Sri Lankan as Colin Cheer, Ben Woodward, Evison and Stroud shone with the ball.
Cheer claimed 2-25 - including the vital wicket of his brother and prolific Boston batsman Jonny for one - while Evison returned 3-21 from four overs.
Woodward then bowled his four over allocation for just 10 runs while claiming the wicket of top scorer Thilanchana as Stroud struck twice to leave Boston on 106-9 off their 20 overs.
Boston then made 138-7 in their second match in pursuit of Sleaford's 154-7 with Kapugedara scoring another half-century.