Robbo
If you google Spanish tax forms you will pick up their site. All you need to do fill in the required details online and they complete the form for you. They send it you and you can take the form to your bank in Spain to make the payment. The cost is £25 for the form. The bank takes if sending the relevant part of the form to the tax office. Spanish tax forms also offer a service of sorting out the payment for you but this costs slightly more.
I see from another thread that you can download form yourself and comets but think you would have to be very sure of your Spanish!
MG