Lesley, perhaps this may help you to understand the education system in Spain
Mar Azul appears to be an independent school and some of the best schools in Spain would be classified as such. It simply means that they do not receive government subsidies to operate, as in the case of the "colegio concertado".
I cannot give you an opinion about the school, but I know Landlady and I would certainly trust her advice based on experience.
Also, the school offers bilingual education, something that cannot be obtained through a public school in Spain, as there are no resources to achieve this.
It is an important consideration, as today's education is becoming increasingly international and English is considered a "lingua franca" that allows students to study throughout the world. Having two international languages such as Spanish and English will certainly give your children very many options in the future.
Also, it is very difficult to maintain native tongue proficiency and expansion unless the student receives proper tuition. Language maintenance at home is very difficult unless parents have relevant knowledge in language tuition, are able to organise a real study program and have plenty of willpower and patience.
I wish you good luck in your endeavours.