The law is probably different on those two points though, as said above there is NO defense for speeding, only mitigating circumstances - which let's face it, are usually just excuses to try and get away with it..... (as far as the courts are concerned).

At the end of the day, if expert evidence did show your speedo was mis-calibrated and there was no way of knowing this beforehand, then you just have to submit that as your mitigating circumstances, proof that you've rectified the fault also would be a good idea , and put yourself at the mercy of the magistrates.....

....but I ain't a law expert.....