As it turns out it was to do with the URL re-writing. I had Quickly implemented a custom form control off an MSDN article to combat the re-writing on the post back. What the control did was to remove the url from the action attribute, however it also removed the
on submit action.
So to solve the problem the following needs to be added to the RenderAttributes override in the custom control, should anyone else be using it.
if(this.Page.Validators != null && this.Page.Validators.Count >=1)