Maybe its an urban myth but I thought there was a bit of leeway to allow for speedometers that aren't exact.
Spedo's can read incorrectly.. but spedos have to be calibrated to only read fast within tolerance.
A speedo cannot read slow.. i.e if you are travelling at 60mph, your speedo cannot read anything below 60mph, it can however read above 60mph (infact anything over up to 15%.
This info is as per the kit car Single Vehicle Approval compliancy.
So, you actually have no excuse if your speedo is wrong.
I got done by a private speed camera van in a 50.. mighty annoyed as it was parked in a hard shoulder on a dual carriage way, but I didn't have the piece of mind to go back and get a photo of it(illegally parked?)
Anyway, I believe they sell their fines to the police, so its in their interest to sell every speeding offense whether it be +1mph or more.
Private speed camera vans are worse that static speed cameras, they don't make considered judgment and they ALWAYS have film.