Many thanks to all of you who replied. I have now checked the Architects Specification and its says 50lts. However 50 lts is not large enough to serve a house that sleeps 8 persons. 👿
I therefore think the best people to go after are the Architects as they presumably belong to a professional body and they shoud have professional Indemnity Insurance.
Could anyone please give me details of Architects Proffesional Bodies in Spain? 👿