Supplier credit note received after year end had been completed and books closed


Ive received a credit note from an energy provider for invoices posted in last financial year, which has been finalised several months ago. How do I treat this? For if I post with current date, it affects my p & l this year incorrectly, If I post with previous year invoice date, it will affect the year end figures. The sum is £2,400 so would be considered material. Help?