Aston Martin completed the top six, with Andre Piccini taking the flag for second in the GT1 class in the car he shared with Oxford-based Darren Turner and Tomas Enge.
Webster moved alongside Manassero with a seven-under 64 which included a spectacular eagle at the par-five 13th, which saw his second shot from a fairway bunker strike the flag, leaving him with a six-foot putt for his three.
With the Enyimba defence waiting in vain for the offside flag, Zitouni found himself unmarked in the six-yard area but headed over the bar from close range.