Update: just received the following reply from Correos:
"Good morning, Jill: In relation to web payment, it may be that if you are managing it from a foreign card, it will not recognize it, and that is why it will not let you make the payment, if you know someone who has a Spanish card, to be able to make the payment, otherwise when you arrive in Spain, you can make the reservation at any of our Offices of the Post Office to make the payment . . ."
As I haven't actually got to the payment page yet it doesn't know what card I'm going to use, and it's my antivirus software that's stopping me. But anyway, one of the other ladies, who lives in the UK (I'm in South Africa), said she'll try, so hopefully it will work for her.