Dai,
If you want to understand the system you should look at the Pope of Slope's website which describes it in detail.
http://www.popeofslope.com/courserating/twoparameter.htmlHow it is applied requires a few days of training and practice and a rather thick manual. There is a lot of judgement in rating the obstacles scores. I guess COVID has probably stalled course rating in the UK in preparation for the new World Handicapping System. You could volunteer to be a course rater for your association - it's a good experience.
At a basic level, a course is rated for a Scratch Golfer who sole attribute is the ability to drive the ball 250 yards and reach a green at 470 yards in two shots. The course is also rated for a Bogey Golfer who's sole attribute is a driving distance of 200 yards and can reach a green in two up to 370 yards. The Slope is a measure of the relative difficulty of the course for a Bogey Golfer and is based on the difference between the Scratch Rating and the Bogey Rating.
There are two factors that contribute to the ratings: the length of the course and the obstacles on the course. The length is the predominate factor by a lot. The obstacles are rated hole by hole on a scale of 0 - 10. and are as follows:
Weight
Obstacle Scratch Bogey
Topography .10 .12
Fairway .11 .09
Recoverability and Rough .14 .15
Out of bounds .10 .09
Water Hazards .14 .14
Trees .09 .14
Bunkers .07 .10
Green Target .09 .06
Green Surface .11 .08
Psychological .05 .03
As a hypothetical example of how it works let's assume a totally nondescript course with just yardage and no real obstacles. The length contribution to the rating is based on two formulas - one for Scratch and one for Bogey.
Scratch Length Rating = (Length/220) + 40.9
Bogey Length Rating = (Length/160) + 50.7
So, for a 7000 yard course the SLR would be 72.7 and the BLR would be 94.5.
For a 5000 yard course the SLR would be 63.6 and the BLR would be 82.0.
The Slope is calculated as the (Bogey Rating - Scratch Rating) x 5.381
So, the respective slopes for the 7000 and 5000 yard courses are 117 and 99 respectively. This is based on just the length of the course.
If the course has obstacles (and all do to some degree) you have to rate each obstacle on a hole by hole basis on a scale of 0 -10, then multiply by the respective weights for Scratch and Bogey golfers shown above and then add them all up and apply the obstacle rating formula.
Scratch Obstacle Rating = (Sum of Scratch Obstacle Scores x 0.11) - 4.9
Bogey Obstacle Rating = (Sum of Obstacle Scores x 0.26) - 11.5
The overall Scratch and Bogey ratings and Slope of the course are based on the sum of the length rating and obstacle rating The overall slope formula is the same as above. As you can see from the obstacle rating formulas you can have some obstacles and have a Slope that is lower than, the same as or higher than the pure length-based Slope depending on the number and severity of the obstacles.
So, you could design a course at 7000 yards that has a number of obstacles and still have a slope of 113. Whether it would be great or not depends on your definition of great.
I wonder if a great course like Pacific Dunes is a great course from the Royal Blue tees with a course rating of 61.5, a bogey rating of 82.5, and a slope of 113? What are the obstacles for the bogey golfer that merit a 143 slope from the Black tees when the course rating is only a modest 73.2 for scratch golfers?