Hi Max
We're not in the business of providing 'loop-holes' for the guilty to evade justice, but in your case it sounds as if you didn't actually commit any offence, and as such an acquittal would be what is known as 'justice', often a foreign concept to our legal system.
Your first step will be to wait and see if a NIP (notice of intended prosecution) arrives in the post. They have 14 days to serve this or it is invalid. If you were indeed traveling at or below the speed limit (often you are given 10%+2mph grace over the speed limit, so up to 46mph in the case of a 40, though they do not have to) then one would hope that checking against the road markings will confirm this, but this does not always take place before the NIP is fired off.
If you do receive a NIP your next step will be to request the photographs of your vehicle. They will likely try to wriggle out of this, potentially denying them to you unless you return the NIP pleading not-guilty, and even then possibly only providing one photograph, quite likely cropped. Knowing the interval between the photos and the road markings will enable your speed to be calculated. If they do not provide these to you at least a week before your hearing then you will ask for the case to be dismissed (even if they could provide them to you on the day you are entitled to a speedy trial, and there would be no good reason why they could not have provided them to you in a timely manner).
If the photographs do not prove your innocence, and you are convinced you were not, in fact, exceeding the limit, then there is evidence for potential inaccuracies in the Gatso units, including unidentified failures since it's last calibration, and timing inaccuracies between the photos, but this is a bridge to cross when you get there, and even then, only if you are convinced of your innocence, since considerable cost and effort is likely to be required.
As a side note, if you do receive the photos and they show that you were not, in fact, exceeding the limit, then you may wish to file counter-suit against the CPS for frivolous prosecution, since there is absolutely no excuse for them not checking the photos before issuing proceedings!
Hope this quick precis is of assistance. Your best bet, if the NIP does arrive within a fortnight, is to check out
http://www.pepipoo.com since they have more expertise in these matters.