“Marry in haste, repent at leisure”, as the saying goes. This case is not quite so dramatic but PAS 121 (in common with most PAS) was written in haste and it shows.
At last though we have a replacement (in fact 2 replacements) in the form of BS 8524-1 and BS 8524-2, both published last month. These have been developed through the full BSI process and are much better.