All the issues are now decided in division one after Peacehaven were relegated on Saturday following their 3-0 defeat at Arundel.
Mick Maskell's men join Littlehampton and Wick in the second division next season after they were beaten by goals from Danny McLaughlin, Jason Wimbleton and Richard Grove.
Victory came as a relief for Mullets' boss Mike Rowland whose side had been sucked into trouble after a poor run since the end of January.
Rowland said: "I can sleep soundly tonight for the first time in three weeks. We've put ourselves in a silly position with a poor run of results but we deserved the victory on Saturday although Peacehaven weren't the strongest of sides."
Horsham YMCA bounced back from two successive defeats to keep their hopes of finishing as runners-up alive with a 4-1 win at Whitehawk.
Joel O'Hara put them in control with a brace, David Oakes' cross sailed into the net from 40 yards and Nick Flint rounded things off with a rare header before Tommy Pattenden got an 89th-minute consolation.
Manager John Suter said: "Chichester also won which keeps it tight and interesting for the last few games. Strangely enough, we played better at Ringmer and lost but it was still a good performance and it is hard to single anyone out, they all played well."
Chi are battling with YM for second spot and they kept the pressure on with a 2-1 win at East Preston thanks to Steve Leigh's late goal.
Matt Smith gave Chi the lead on 37 minutes after being set up by Marcus Priest but the home side levelled six minutes later through Simon Clayton. The winner came in the 90th minute when Leigh fired home from a corner.
Chi boss Adie Girdler said: "We have both got three to play. They have got two hard games and we have got two hard games. It will go down to the wire."
Hailsham Town spoiled Sammy Donnelly's first game in charge at Three Bridges by grabbing a 2-0 win at the Jubilee Field.
Second half goals from Simon Stevens and Scott French were enough to give Hailsham the points to the delight of their manager Brian Dennis.
He said: "Sammy Donnelly has taken over and has a few problems with players leaving but he had them geed up and motivated so I am delighted."
Donnelly said: "I would have loved to have won but you have got to see beyond that.
"There is plenty of room to manoeuvre but there were five changes from the last team and that counts. The great thing is I have six games remaining to look at the players and look at their character."
Champions Burgess Hill won 1-0 at Pagham thanks to a 16th minute penalty by Matt Geard.
Hillians boss Danny Bloor said: "After winning the league in Tuesday, it could have been a case of 'After the Lord Mayor's Show' so I am really pleased."
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