Where a given element (such as sector code) is used with a vocabulary provided by the reporting organisation (see code 99 on the sector vocabulary), I propose that the presence of a URL attribute that links through to the external codelist used should be made mandatory by the schema.

Unfortunately xml schemas don't have a facility for making one attribute of a given element mandatory based on the presence of another, so we would need to work out the logistics later - possibly using rulesets, should people be in favour.