Do you dispute that you were exceeding 70mph? If you were doing in the region of 80 or more, then the officer's witness statement would be enough evidence, as long as they come across as credible enough to tell the difference between 70 and 80+ (which any traffic officer would). If you really think you were doing less than 70, then what do you plan on using as an argument? Otherwise you'd only be debating how much over 70 you were doing, and you've been given the best offer you'll get.