Finn O'Mara's goal two minutes before half-time was enough for Gary Elphick's team to win the Sussex derby - and Cray Valley PM's failure to beat Corinthian meant United leapfrogged them to lead the table.
It was their fifth straight league win and puts them in a superb position as they enter the new year, which they'll begin with two games in three days - at home to Lancing on Saturday and away to VCD on Monday.
Elsewhere in the Isthmian south east, Lancing and Chichester City fought out a 0-0 draw at Culver Road, a result that leaves the Lancers 18th in the table and City seventh. See a gallery of pictures from the Lancing-Chi City match by Stephen Goodger here.
It was also honours even on derby day at Burgess Hill Town, where the home side took a 57th-minute lead through Lewis Finney only for Haywards Heath to claim a point through Tom Gilbert's 83rd-minute equaliser. Heath sit fourth, Hill tenth.
Three Bridges are up to 15th after a good 4-2 win at Sevenoaks. Two goals apiece from Dan Perry and Curtis Gayler earned the points.
In the Isthmian premier, a Joe Taylor double and Deshane Dalling's late strike earned Lewes a 3-1 win at Horsham, who had only Shamir Fenelon's goal to show for their efforts. The Rooks are fifth, while the Hornets slip to 13th.
The big Bognor-Worthing game at Nyewood Lane - washed out on Monday - has been rearranged for today (Tuesday, 3pm).