Ok, an update..
the document from the Ajuntament normally takes a week to issue - obviously no use to anyone.  They can issue a document called a 'Volant historic de l'habitant' which, in the case of Roquetes, they will not charge for.

The 'comissió de seguiment' for all of the Terresdel Ebre (Baix Ebre, Montsia and Terra Alta) is in Tortosa. It is the Delegación de Govern in Carrer de Montcada 23 which is near the Library, open from 9am to 2pm.