When it was brought out in 1982 BS5975 was an easy to use guide to designing and erecting falsework. BS5973 (1993) was the companion book for scaffolding, again easy to use for designing and erecting scaffolding. BS5973 was withdrawn to suit the Eurocode drive and replaced by EN12811-1 which was much appreciated by academics and researchers with PhD level education but pretty much useless to everyone else. That led to TG20 being put together by NASC to get back to something that was more useful to the average designer. BS5975 escaped being withdrawn but did change from 2008 to 2011 from being 125 pages of generally useful information to 215 pages. The increase was in every section but the bit on loads and designs went from 37 pages to 69 pages and a big section of about 15 pages went into procedures that had previously been summarised in 4 pages. The latest version of BS5975 is 250 pages.
Twf have produced a 'complementary' document to explain what all the words in the procedures part of BS5975 actually mean. Most of this is aimed at managers in construction organisations who, despite the idea of a design brief being suggested in the Bragg report of 1975 and formalised in BS5975 in 1996 have still not managed to understand how to produce a design brief. A very small number have but they are really the exception. I doubt that adding more documents for them to read will change this for the good.
Download the latest guide if you have time to read it but it is no more clear enough about what constitutes an acceptable design than the H & S act is and so every organisation will have its own idea of what they need. If you want to pick up what EN12811-1 requires in the way of calculations, follow section 6 word by word and make sure that the calculations have addressed every line. At least then you will be basing the calculations on the base authoritative document even if it has no legal status.
Sorry about that rant - don't start me on risk analyses!!