I use Sainsbury's (Highland? Scottish? can't remember) quite often, it's like 55p for 2l. I think most bottled water will be better to use than filtered tap water just for the consistency – a filter deteriorates over a short amount of time so god knows what TDS it'd read from one day to the next. I use one for drinking water but haven't had great results with coffee.
Or take an empty bottle to a nice cafe with an RO tap, fill up and leave a little tip.
I use Sainsbury's (Highland? Scottish? can't remember) quite often, it's like 55p for 2l. I think most bottled water will be better to use than filtered tap water just for the consistency – a filter deteriorates over a short amount of time so god knows what TDS it'd read from one day to the next. I use one for drinking water but haven't had great results with coffee.
Or take an empty bottle to a nice cafe with an RO tap, fill up and leave a little tip.