\\ I take it the prevailing limit was 50mph or less as enforcement does not begin until (Limit + 10% + 2mph).//
Is this written in law?//
No it’s not written in law. It is the official guidance which was provided by the Association of Chief Police Officers. When that body was abandoned in 2015 the guidance was adopted by its successor, the National Police Chiefs’ Council. It is here:
https://www.npcc.police.uk/2018%20FOI/Operations/030%2018%20Speed%20Enforcement%20Guidance.pdf
The document explains the principles behind speed limit enforcement (including the reason for the "tolerance”) and the table of thresholds is shown in paragraph 9.6. It is followed by all forces in England and Wales and has also been adopted by Police Scotland.
//I attended a speed awareness course about sixteen years ago and the first thing we had to do was to tell everybody the speed we should have been doing…..etc.//
That’s interesting. I subscribe to a number of motoring forums and very often there are questions from drivers who say they have seen enforcement below the NPCC’s guidance thresholds. They are usually asked, so that people know where the guidance is not being followed, to post a copy of their paperwork. I can say that, without exception, none has been able to substantiate their claim. Very often they don’t respond at all or, if they do, it is “Sorry, it was 35, not 34” or similar. Whilst 1mph over the limit is an offence, the NPCC document explains why enforcement should not normally begin below the threshold.