You need to write to the Department to ask for a card which you then give to your employer, as the instruction to stop deduction NI from your pay. This is what my Payroll Department advised me, so this is what I did:
"You need to obtain a 'Certificate of Age Exception' from your local Contributions Agency and send it to the Payroll Department before your 60th birthday. "
This was a couple of years ago so the qualifying age may have changed since then, but the same principle applies.