Thanks for the replies.

Iron isn't listed on the label so unfortunately I can't check that,
but I'll look into the PH of the different bottles, and at filters.

One of the bottles does leave a film on the cup and sometimes a layer
along the top after brewing. I presume this is the 'somewhat basic'
water described by Eric J.

I'm getting an OK tasting tea, but nothing like as good (in my
opinion) as the tea made by the Glasgow water I'm used to, despite
being city water its some of the best tasting and best tea making water
I've come across.