If it helps (!) I'm in a similar boat with car tax - haven't been out since March and don't know when we're next likely to be there. Anyway, I was redirected by the Santa Pola town hall "rentas" department to SUMA, who they hand things over to when the tax becomes overdue - had an exchange with them, having given them details of car registration/NIE. The fine for late payment was just over 3 Euros, and paying the whole thing on line was easy. Next problem when we eventually get there will be the out of date ITV........!!