I would like to dispute this charge. Please can you advise me on how to go about doing this.
As above, disputing it is simple. The difficult bit comes after the police have presented their evidence to the court that you had exceeded the speed limit.
Average speed camera systems are very accurate. They take two timed photographs of the vehicle and the distance between the two cameras is known. So with the time and the distance known it is a simple calculation to determine the vehicle's average speed over the stretch. If your average speed was above the limit, you must have exceeded the limit in either all or part of the stretch.
So when the police have done their bit, what are you going to do to counter their evidence?
Failure following a trial is costly. You will pay an income-related fine of half a week's net income, a surcharge of 40% of that fine and prosecution costs of around £650. So an income of £500pw will see you pay £1,000.