I understand that it is not as simple as paying off the debt. There is a separate process to cancel the mortgage with the bank. Can anybody give me an "idiots" guide to the procedures please.

Not so complicated.
1. You need a certificate from the bank confirming the mortgage has ended
2. Based on the above get a notary to issue an "escritura de cancelacion" which will alos need to be signed by the bank - the notary should arrange that.
3. Do Form 600 for Hacienda - Impuestos Sobre Actos Juridicos
4. Present all the documents to the Registro de Propiedades where it should be officialy recorded

You will need to pay the Notary and the Registro

As a check you can get from the registro de Propiedades a "Nota Simple" -a document which you probably got when you bought the home which tracks all the history of the property.

As a PS - the bank is really obliged to do all this it just takes time.

There are two ways to do it - the bank doing everything (between €200-400, less complicated) or you doing it yourself (about €125, more complicated). This link explains the two procedures below, in Spanish:

Levantamiento o cancelación de la hipoteca: trámites y gastos

Thank you Loafing Along and DLC for your help