Hornchurch captain Billy Gordon hailed his side's success in the Vitality Club T20 Cup area final as a 'brilliant day'.
Gordon hit 51 off 43 balls as they posted 150-5 against Potters Bar, putting on 80 with Adeel Malik (31 not out).
And Malik nabbed 2-11 as Bar came up short on 148-5, before Gordon (67 not out) and Jalpesh Vijay (61 not out) led Hornchurch to 187-3 against Horsford.
Malik (4-20), George Hankins (2-12), Gordon (2-22) and Ronnie Saunders (2-29) combined with the ball to seal an 88-run win and a place at the Southern final on August 28.
They will now face Bexley, while Banbury take on Bradninch in the other tie at Basingstoke & North Hants, with the winners meeting the Northern champions.
And Gordon said: "Sunday was a brilliant day, with us hosting it. The weather was perfect, we had a pop-up bar outside and music and all the teams were good as gold.
"Potters Bar are a good side and we did excellent to beat them. They needed 10 off the last over and I was pleased how I bowled. I was just making sure not to overstep and they hit the last ball over cover for six, but it was a brilliant win.
"Once me and Jalpesh got going, we said we'd try and get to 15 overs and go from there. The last five overs went for quite a lot.
"Jalpesh has shown glimpses and T20 absolutely suits his game, he is very aggressive.
"It was brilliant. We never expected to get through and I don't think the other teams did. They were young and athletic and looked at us and probably thought we had no chance, so were a bit shocked!
"[Marc] Whitlock came back from his holidays to play, then went back!"
Sunday's success came after Hornchurch had been frustrated by Chingford in their Hamro Foundation Essex League Premier Division content a day earlier.
Gordon won the toss and saw Jamie Sorrell hit 18 fours in his 111 from 137 balls, while Hankins clubbed 51 off just 29 balls, with four sixes and five fours.
Malik added six sixes and two fours in a 60-ball 68 as the visitors piled up 347 and Gordon added: "It was a really good toss to win. It's really flat over there and it was great to see Sorrell get his first Premier hundred, which he was really happy about.
"George batted superb again and Adeel lost a few balls and we were really pleased to get a massive score."
The hosts reached 95-1, but then lost five wickets for just 14 runs as Gordon (4-62) and Malik (3-66) made inroads.
Chingford captain Andrew MacGregor (64) held up the charge, though, and his side closed on 197-8 to leave Hornchurch in fourth place with four games to go.
"They've got some good players and it's a small ground where you can score quick but we did well to get them eight down and put the pressure on, with no Whitlock again," said Gordon.
"We were just another bowler short again. We were saying if we'd had an overseas all season we would've been in with a shout."
Only four points separate Hornchurch and seventh-placed Chingford, as Gordon's men head to lowly Belhus on Saturday.
And the skipper is hoping they can avenge the 121-run loss suffered when the sides met back in mid-June.
He added: "It's been tight all season, apart from Wanstead. We're 60 points clear of relegation, but we need a good result.
"We want to finish in the top four to get in the National next year and Whitlock will be back so we will be a bit stronger."
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