There is no mention of the delay in onset on the NHS website:
http://www.nhs.uk/Conditions/Chronic-fatigue-syndrome/Pages/Diagnosis.aspx
they only say:
".....the tiredness:
gets worse after activity or gentle exercise, such as a short walk
doing exercise or concentrating makes your symptoms worse"
but they do link to NICE so I suppose they think it's covered.
Is it my imagination or has the NHS page been updated to include these as I don't remember seeing them before?