Jools Ive  PM you, but if you've had an overload which caused a fire  it can really be one of 3 things. Incorrect wire size for the circuit being used or an incorrect  value MCB ( minature circuit breaker) fitted or the most unlikey a defective breaker.

As The spanish use spurs rather that ring mains correctly sized wiring and MCB will always trip well before wiring start to get so hot it causes afire. Even a direct short would trip the relative MCB in milli seconds.  So even if you overloaded a correctly wired crcuit all that should happen is the spur crcuit trip  should switch. Also consider getting an independant assesment  (once you find the elctrian)done on your wiring apart from the builder and his sub contractors as you are dead right  even if you accidently over load or have a fault anywhere in you electrical crcuits, fail safes are in place to prevent such fires. Me thinks negligence is a foot !