It came down to the final game of the season for Barum, who had seen a 10-point buffer between them and the foot of a table in February cut to just two points by a resurgent Bishops Cleeve.
A point would have been enough for Barum to ensure the matter wasn’t out of their hands.
In the end Bishops Cleeve’s 2-0 defeat at Yate Town meant Barnstaple’s defeat to Supermarine didn’t have any wider consequences.
Barnstaple co-manager Kevin Darch said: “I’m just pleased for the players – they didn’t deserve to finish bottom of the league.
“We went off the rails in the last five games and we really should have had it sewn up.
“They’ve had it tough this season and I think they have done well. It was a bit closer than it should have been though.
“It’s a very young side, and really only Niall and Jed have been played at Southern League level, so they’ve been punching above their weight all season and I don’t think enough people understand that.
“I’m just delighted, because they’ve acquitted themselves really well over the season.
“We played really well in the first half on Saturday, we had got our shape and limited them to one chance – I was really pleased.
“I thought we would nick something. But the first goal goes in five minutes after the break and then panic set in.
“We just left ourselves far too open for 20 minutes and they got two more goals.”
Barnstaple had held firm in the first half at Swindon, but a quick-fire brace from Bradley Gray in the first eight minutes of the second half tilted the game in Supermarine’s favour.
Gray opened the scoring in the 49th minute with a well-taken goal, flicking the ball over Nathan Cooper’s head before side-footing past Rob Holmes in the Barnstaple goal.
He was in the right place at the right time four minutes later, finishing from close range after Holmes had tipped a Zac Westlake header onto the post.
Gray was provider for Callum McGhee-Parsons with 12 minutes remaining, flicking a header into the path of his strike partner, who fired in.
The result sealed a play-off berth for the hosts, while Barum were left waiting on the final score at Yate.