I think Essjay was thinking of a water softener, when it seems like you are referring to a sediment filter.
Since it appears you are in the UK, the Pozzani filter may be your best bet (I don't believe it's available in the US). Also, I don't know about cartridge use life or if it can be recharged.
I have high nitrates in my well water most likely due to the 95 acre farmers field across the road that gets ample fertilizers! I originally opted for the (now discontinued) API Tap Water filter that produced deionized (DI) water. However, with my water, the cartridge was spent after only about 50 gallons and considering the cartridge cost it just wasn't worth it (as I could buy spring water cheaper!).
I had used API Nitra-Zorb pouches here in there in my filter and I wondered....what if I emptied a Tap Water Filter cartridge and filled with Nitra-Zorb? I ran the idea by API Tech support and they informed me that they just didn't know as the product hadn't been tested that way. They went on to tell me that the use life was reduced due to detritus coating the resin. (but that wouldn't be the case with clean well water!)
(Nitra-Zorb is a polymeric resin that adsorbs, ammonia, nitrite, and nitrate and can be recharged many times with ordinary non-iodized salt water.)
I went ahead and did it and it works perfectly. I produce about 250g before I need to recharge and I've put over 3500g through the filter and it's still going strong!
Along with pre-filtering tap water, you'll also want to take other measures to minimize/reduce tank generated nitrates! ....I posted recently under a thread about sponge filters.