There seem to be a lot of grumpy tourists surfing the web after their holidays in search of a method to pay their fines. Strangely, the Spanish don't make it easy! I finally cracked it and share the steps below, some of which were the result of a call to the Spanish authorities on the number given on the fine notice, but the links came from surfing the net. Please take care when using the links/urls to ensure you are on the official pages!
Click here: https://sedeapl.dgt.gob.es:7443/WEB_Sanciones/jsp/sincertificado/identificacionPagador.jsf [note this is reached via https://sede.dgt.gob.es/es/tramites-y-multas/alguna-multa-en/pago-de-multas/ , which in turn is reached by clicking the union jack “Is there a fine?” link at http://www.dgt.es/es/].Drive safely ;-)
When the online form comes up, complete as follows:
Under "Document type", click the drop-down menu and click “passport”
Under "Document number", type 000000 - six zeros
“Date of the Report” means the date at the top of the fine notice in the box marked “Fecha Denuncia”
“Record Number” means the number in the top right of the fine notice in the box marked “N. Expediente” – with all dashes and dots removed
For the amount, you must put the total amount of the fine, not the discount for early repayment - the site will then calculate any discount and display it in the next screen, which will also give you the opportunity to pay by debit/credit card.