www.pepipoo.co.uk may help you.
As for being done for 31, not allowed as speedos are allowed 10% inaccuracy. In reality they work on 10%+2 so you are looking at 35 in a 30 zone before they will do you.
There is something that there must be a sign within 1km of teh van warning you and usually there is a temporary sign that they leave in teh back of teh van. This is meant to be placed 100m before the van.
However, in order to try and get off through one of these loopholes you have to take it to court. If you lose you get far more than £60/3pts for wasting the courts time.