When Gas Natural launched its surprise €22.5bn ($35.9bn) bid for Endesa, Spain's largest electricity group, earlier this month, it was most certainly braced for the ensuing public debate.

Not only was the bid brash - coming from a company half Endesa's size - but the pre-arranged sale of up to €9bn in assets to Iberdrola, the country's number two generator, automatically raised questions about concentration in the Spanish electricity and gas markets.