This is true - but it takes a lot of proving, otherwise everyone would just tear them up and say they'd never arrived.
If it was 5 weeks ago you would've received it by now.
dh does quite a bit of work on Police enforcement systems. A lot of forces are still using traditional cameras with film inside them and moving the cameras around from one gatso unit to another. The gatso unit flashes even if there isn't a camera inside - and the film runs out quite quickly which is why they only usually flash at a relatively high speed.
So... you're safe for now, but BE WARNED - lots of Forces are moving to digital technology which catches EVERYONE so the gatso can be set to go off at a lower speed (often 33mph)
IIRC the envelope it comes in isn't stamped or franked, it's printed (I had one the other day ) - so I'm guessing there is some sort of monitoring system to see what goes where and when - if they know it went into your sorting office on a given day you're going to struggle to prove it wasn't delivered, aren't you?
The people running the course said that you were allowed to do 35mph to allow for differences in speedometers. They didn't mention any areas that don't apply that rule and everyone (about 20 of us) on the course had all done 36, 37 and 38mph and come from all areas of the country.