Loads of great info there guys, thanks. I am riding a frame that is probably a bit too small for me, but I preferred the handling to the larger bikes. The bike feels fine at the beginning and it is only towards the end that I start to get the problem. I thought that lengthening the distance detween the saddle and the bars would allow me to have a flatter back and reduce the pressure on it. A lot of you are saying that it could be my core/poor strength/fatigue that is causing my issues and I think you may be on to something there so I'm going to go away and work on that and see how I get on.
Thanks again.