Langleybaston - thanks for that interesting bit of background info on turbulence.
You said
I have searched Google but cannot find chapter and verse for the 56 group coding.
I suspect from your background that you may be looking for something more technical (i.e. how the model for generating the forecast turbulence works), but in case you are just looking for the meanings of the codes, this is from the "US Air Force Aircrew Quick Reference to the TAF and METAR codes"
"If forecasted, the turbulence code will be prefixed by the number 5, and will follow the cloud or icing group. To decode, follow these instructions: 1. Look for the turbulence designator “5” that follows the cloud or icing group (520004).
2. The next digit will determine the intensity (520004). See Figure 4.
3. The next three digits will determine the base limit of the turbulence layer in hundreds of feet AGL (520004).
4. The last digit will determine the turbulence layer depth in thousands of feet (520004), so add this value to the base height to determine the top limit of the turbulence conditions.
In the above example, the turbulence forecast will read, “occasional moderate turbulence in clear air from the surface to 4,000 feet.”
Figure 4. Turbulence Intensity Decode Table
0 - None
1 - Light turbulence
2 - Moderate turbulence in clear air, occasional
3 - Moderate turbulence in clear air, frequent
4 - Moderate turbulence in cloud, occasional
5 - Moderate turbulence in cloud, frequent
6 - Severe turbulence in clear air, occasional
7 - Severe turbulence in clear air, frequent
8 - Severe turbulence in cloud, occasional
9 - Severe turbulence in cloud, frequent
X - Extreme turbulence
Note: Occasional is defined as occurring less than 1/3 of the time"
You also said you wondered what had changed in 20 years regarding low-level terrain-induced turbulence. I can only assume that they have developed some new modelling of what they think happens - I am just not convinced from experience and observation that they yet have a model that is "fit for purpose"!