The builders office must have the forms and I understand they are in triplicate. (Ask for Hoja de Reclamacion) Your complaint has to be written in Spanish and the company has to sign it. The company retains one copy, the OMIC has a copy and you retain a copy. I am sure your local office would be Murcia. Their website is MurciaRegion.com – you could email them.
All the lawyers who post on this forum are on holiday at the moment.