Looks like most bottled water is ok (and like tap water, doesn't need to be boiled from 6 months). Found this (probably far more info than anyone wanted, sorry
):
Bottled waters, other than those labelled 'natural mineral water', are expected to conform to essentially the same standards as the public water supply and they are therefore suitable for giving to infants or for preparing feeds. As with tap water, bottled waters should be boiled and cooled before using to make up infant formula feeds.
'Natural mineral water' is covered by less comprehensive regulations than tap water and may contain higher concentrations of solutes such as nitrate, sodium, fluoride and sulphate which might lead to solute overload if these waters are given to young infants.
The tap water that we drink every day contains only chloride and flouride - all minerals are removed during the treatment process. However, with mineral water the composition is unchanged - extra minerals are neither added nor removed from it.
Concentrations greater than the following should be avoided for infant feeding according to the Department of Health guidelines:
Maximum mineral concentrations for drinking water (UK Department of Health)
Sodium 200 milligrams per litre (mg/l)
Nitrate 100 mg/l (best below 50 mg/l)
Nitrite 3 mg/l
Sulphate 500 mg/l
(From https://www.babymilkaction.org/pages/bwater.html#3)