Emma
I'm afraid the ESA legislation allows a reassessment as early as three months after a previous assessment. Unless you can get the form withdrawn I'm afraid you have no choice but to complete the form. Treat it as a new claim and complete the form in detail and supply evidence in support even it was sent with the last form.
Try contacting your MP about the new form, but be aware of the form's return date.
Contacting your MP Gordon