i followed the advice with online chat line advisor and it worked too. YES! FOLLOW THIS ADVICE!
it took 15 mins to complete the enquiry, receive ref number and details of bank account to make payment.
very easy- so much better than trying to get connected on the phone line.
just make sure you have your personal details ready, NI number etc, a list of the years and payments you want to make, and the total you want to make.
then just need to wait for the payment to be made i guess!