If we get told not to drink the water due to high mineral content what do they use when brewing dorada or tropical?

Tenerife water and Gran Canaria water respectively, where they are brewed. Since they don't, as far as I am aware, desalinate sea water to brew them, there should be no issue.

I find I get a sore head the next day if I drink more than 6 pints, probably due to the high mineral content of course.

No Robert, think about it. You feel fine, but a little spaced when you go to bed, but horrible when you wake. It's the sleep that gives you the hangover

I think you've got something there. Next time I'm going to stay up all night drinking and bet I don't get a hangover in the morning.