Janet,

Best thing to do - put 4x4 inch timber or cement posts in either end of your gap. These need to be sunk 2ft into the earth and set in with a 3-1 ratio of ballast-cement. (this can involve some heavy breaking out of existing posts, if rotten). Otherwise use existing posts. Once posts are in, buy 3 x 9ft lengths of 'arris' rail (triangular fence beams) and 3 left and 3 right arris rail brackets and a bag of 4 inch nails. Nail brackets to posts at desired height of rails. Nail rails into brackets. to be continued!....