NPPF and Existing Employment Sites - Public forum - Planning Advisory Service (PAS)

NPPF para 22 states that planning policies should avoid the long-term protection of sites allocated for employment use where there is no reasonable prospect of a site being used for that purpose. Whilst the position on allocated sites is unambiguous, it is unclear whether this approach applies to areas identified as existing employment areas which often make up a significant, and often more readily deliverable, element of the supply of employment land and premises. How have others approached this issue?