Ball double seals Sidley win
The Blues opened the scoring in the 15th minute through Mark Ralley and Owen Ball added another nine minutes later.
Lee converted a penalty for the home side on the hour mark before Ball scored his second with ten minutes to go.
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Hide AdIn the dying minutes Ford pulled one back for Selsey but it was too late to stop United going through.
Bexhill Town slipped back into the bottom three of the Sussex County Third Division after losing 3-2 to high-flying Haywards Heath Town.
Three first half Heath goals from Hatt, Wright and Elphick were enough for the points despite Steve Galt and Conrad Sparks netting consolations.