Southwell struck on 48, 57 and 73 minutes to seemingly put United on course for a routine win until sloppy defending allowed Joe Conchie to cut the deficit six minutes from time.
Keeper Sam Vince was sent off for violent conduct in the moments after conceding and United had to rely on Notts County loanee Greg Tempest to don the gloves and make two excellent late saves.
PIC: Tim Hoff.