Hi Mozza,
The procedure for registering online with Endesa goes like this:
Go to this link: http://www.endesaonline.com/ES/hogares/oficinaonline/formaltaoficinaonline1.asp
Choose option "Soy propietario de viviendas" and click on Aceptar
On the next page in Empresa select Sevillana
In Contrato put in the number of your Sevillana contract probably 11 numbers starting with 9703 if it's like mine
In the next line put in the number of your last bill, which is about 16 characters starting with 00S in my case
Under "Datos del Titular del Contrato"
Nombre is your surname (the name on your contract)
Apellido 1 &2 are your first and second names
Tipo de Documento : choose Passport
No. de documento is your passport number
Telefono de contacto: Telephone number in Spain (obligatory)
Email: your email address, also obligatory
Then you type in a user name, min. 6 characters
Then a password, again min. 6 characters
Then repeat your password
Leave next option at "No"
Click on box to accept the terms (He leido Acepto los terminos legales)
Then click on Aceptar
I think that is the end of the process - you should get an email confirming your registration after a day or two.
As you can see, in order to get through this process you need to have some information from your last bill and I think a Spanish telephone number - I'm nearly sure that it won't accept an international number.
Hope this helps