Bradcot awning - help needed !!

 We are new to caravanning and have recently been given a 2 berth Bailey Ranger (1999). It has a Bradcot Awning which seem's to be quite old with fibreglass poles. We have had several attempts at putting the awning up but keep finding we have too much slack and a sagging roof. It even looks like the awning canvas is curved at the corners but the pole framework is a typical angular design! Are we doing something wrong or is it the wrong awning?!! We are supposed to be going on our first trip tomorrow in the wind and rain and have 2 very excited little boys so any advice would be much appreciated.