Hi soph,
I agree with Paul regarding flushing etc.
The filter Paul mentions is very good for drinking water and you actually finish up with less limescale in the kettle. They are available from: www.generalecology.com
I fitted mine in less than an hour an dwhen I renewed the filter in the autumn, there seemed to be so much fine sand like stuff on the outside of the filter that I was amazed it was still letting water through.
I started off with a new stainless steel tank 2 years ago but I was amazed at the amount of stuff the filter got out.
If you can get to a boat show, these filters are a decent bit cheaper than buying them from General Ecology and it gives you an excuse to go to a show.
Happy boating,
Colin