Written by John Kruse, one of the leading experts on Bailiff Law, this consumer friendly guide is essential reading for anyone who comes into contact with a bailiff.
The book is easy to understand and clearly explains the rights
a bailiff has, and also what they cannot do when collecting debts and repossessing goods etc.
If your speedometer was faulty, then I would strongly suggest that you don't use this as an excuse - you will, in effect, be admitting to a further offence of driving an unroadworthy vehicle.
I can't see the issue myself. You say that you were caught at 39 mph and thought that it was a 40 mph limit, when it was, in fact, a 30 mph zone. Even if your speedometer was absolutely accurate, you would still have been speeding.
By your own admission you were 9 mph over the limit...
Your only hope of getting off is a procedural error on the part of the authorities.