Also does your tap water system go through a water softener or purifier, something like that. Have a look under sink cupboard and see if there is sort of cannister connected to tap water pipes.
No Kirky i dont,well i have a purigen pouch in the filter...Charlie there is no canister on to the taps etc just normal plumbing under the sink....i used the purigen,as my WP were all over the place etc.
Yes, it's fairly uncommon, but there are places in the UK that have very low nitrate levels in the tap water. I'm in Cornwall and mine is usually 0; it does go up to 5 or 10ppm on occasions.