Few random bits:
They must serve a Notice of Intended Prosecution (NIP) on the registered keeper of the vehicle within 14 days of the offence. They can issue it on day 14 at the latest, if it gets to you on day 16 its still valid.
So first thing is to wait 14 days - no letter means no prosecution (if its a company van the letter may go to them).
Speed, and allowances:
By law, all new cars speedos must read 5% + 2mph over the actual speed.
This means that at 26mph actual, your speed is showing 29.3mph. So when you are poodling along at exactly 30mph indicated, your speed is a fraction over 26mph.
So in the original scenario, your speedo indicated 44mph, but your actual speed would have been 40mph.
This relies on the vehicle being to manufacturers spec regarding tyre size, so if you have put on larger tyres, the speedo will now under-read, which tends to make the actual speed equal indicated (ie it wipes out the difference).
The generally accepted 'allowance' of 10% is a simplification of the 5%+2mph figure, and is taken as a pragmatic guide by traffic officers - its easy to explain 10% allowance....
In addition, some forces have their own guidelines for Motorways - under an indicated 80mph is not likely to get you a ticket, to establish a sort of common agreement, motorways are policed in 'clusters' where they cross several force boundaries ie around the Midlands, so they have a common Motorway policy.
So a form drops through your letterbox saying you were doing 34mph in a 30 zone......one of the principles for prosecuting what at first looks to be a minimal overspeed relates to the first point regarding how speedos are calibrated. In this example, the actual speed is 34mph, so the speedo would actually be showing an indicated 34+5%+2mph ie 37.7mph. The consensus is that if your speedo is showing 37mph and you are in a 30mph zone, as a competent driver you should notice and slow down accordingly.
The speed awareness courses - ACPO (Police bosses) policy is only one offence in 3 years in England and Wales (Scotland has different laws).
The reason you must attend the course within a certain time limit is that the time limit for prosecution is 6 months from the date of the offence, a summons cannot be issued after 6 months (its called malicious prosecution). This relates to most, but not all, motoring offences, specifically those that can only be tried in a Magistrates Court.
If you modify your vehicle and the speedo is no longer accurate, then you must have it recalibrated. I'm pretty sure this is covered by the Construction & Use Regs. The recalibration can be to actual speed, not the 5%+2mph figure.
Not knowing your true speed ie modifying and not recalibrating, is not a defence. Nor is unknowingly speeding because the vehicle you are driving does not have an accurate speedo, it is always the drivers responsibility, regardless of who the car belongs to - although if it is a company vehicle the employer can also be prosecuted for many offences, as the vehicle is being 'used' on their behalf.
Wear on the tyre, and low air pressure, are not defences, as they make the tyre smaller, not bigger, so your speedo would be over-reading as the diameter gets smaller.
Speedo's and odometers are calibrated differently. The odometer will read the correct distance, regardless of the speedo over-reading.
My 255's give me an under-read of approx. 5%, so at an indicted 30mph my actual speed is 31.5mph, so I tend to drive just under the speed limit, and never more than 65mph indicated (where allowed, not through villages etc!).
My odometer under-reads by 9%.
Both of these figures are consistent with the increase in tyre size.
Graham - you mentioned under-reading at low speed and over-reading at high speed - as both the tyres and the speedo remain a consistent size (I hope..) the difference must be due to some sort of wear in the bit in between, either the cable or the bits at each end they go into.
Pete