Hastings and Bexhill were unable to make it a double Sussex promotion.
The Sussex One runners-up lost 10-0 away to their Kent counterparts Ashford, who claim the remaining place in London Four South East.
Hastings defended superbly but were caught out right on half-time when temporarily reduced to 14 men through injury. A second try after the break settled matters.
Meanwhile, Hove beat Newick 7-5 in the final of the inaugural Lewesian Cup, a 15-a-side competition for senior second and third team players at the Stanley Turner Ground.
Eight teams entered and played a good standard of rugby.
Lewes A won the plate final after a 26-6 final defeat of Harbury, a touring team from Warwickshire.
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