Thank you VERY much for all the tips, every one of them make complete sense. My frame is aluminum but I think it may be bowing slightly, probably from my having the quilt too tight.
The he lady I bought it from had made a couple of cloth leaders, which worked well, and I thought I was pretty careful making sure the quilt was straight, but will definitely pay more attention next time.
The spray starch is a brilliant idea and I will also double check the backing is on straight next time.
Thanks agin, much appreciated
Carol