Hi John,
Things always get confusing at this time of year as the annual 'uprating' letters are being sent out, giving details of ESa payments for the next year. The only way you will know whether this is an uprating letter or decision letter is to contact DWP and ask.
ESA is an open-ended award, so there are no end dates on the decision letters. You can find out when to expect reassessment by asking DWP for the 'prognosis' from your computer file.
Nothing on this board constitutes legal advice - always consult a professional about specific problems