Hi Bod
I also live in Norwich, i have a 1000ltr ibc tank and a 300gpd ro, my tap pressure is only around 60 psi and that fills the tank in 1 1/2 days.
My tap reading is around 395ppm and the ro takes that down to around 4ppm after the ro then down to 0ppm after di.
I only have to change the di resin around 2 1/2 - 3 months and i change the pre filters every 6 months, so the basic cost of running the 300gpd ro is very minimal.
Hope this helps
Regards John