I initially did some google research and found a couple of useful links of which this was the best. You need to scroll down because the first part of his instructions pre-date the option to pay over the internet. The correct procedure is outlined in the postscript and in the comments.
I followed these instructions and got to all the right pages but it would not except the ID number. I kept trying until I gave up and called the Travel Department (DGT) on 00 34 902 508 686. I gave the women the number and she said the ticket was not from DGT but from the local Catalan Government (the clue was that is said Servei Catala de Transit on the summons).
So I found their site and after a lot of looking around I figured how to pay online. The site is www.gencat.cat/ovt but rather than repeat all the steps I went through to find the right page there is a link here that will take you straight to the payment page.
1. Follow the link
2. Once you get on to that page enter the Numero de Expediente from the top right of the summons. Enter this number without dashes or slashes
3. Then for ID select Altres (others)4. Then put in the ID number from the bottom centre of the summons (it will probably include some version of your name)
5. Click cerca
6. The next page shows the details of your fine. At the bottom right is a button "pagament amb targeta" pay with card, click this.
7. A new window pops up, fill in the card number and expiry date then press continuar
8. Next window fill in the short code on the back of the card, click continuar again
9. The final window should show that you have made a successful payment. Look for the big green tick.
10. To print a receipt click on imprimir
The fine was very reasonable. Because I paid quickly I got a 50% reduction and only paid 50 euro for doing 150 kmh in a 120 kmh zone. That's half what it would be in the UK and if I was caught doing 150 kmh in Norway they would have sent me to jail - no really they would. So all in all don't mind paying it. Firstly the law is very clear, you know the limits and if you chose to ignore them you should not moan about the consequences. Think of it as an extra tax, you want to drive quickly then you have to pay the speed tax from time to time (like all taxes its higher in Norway!). Also the Spanish economy is buggered so any help we can give them is useful and should not be resented.
POSTSCRIPT Jan 2013
It seems that the link doesn't work anymore but someone has posted in the comments that you can call 0034 902 400 012, select option 2 and then get a person who speaks English and will take your card details. I have not tried this. If anyone finds the new link and sends it to me I will also update the directions above.