Hi Steve,
See my post on page 1.
I have a three stage reverse osmosis system under the sink; it takes up about 2 foot width by about 9" depth at the back of the under sink cupboard in the corner that you wouldn't use anyway for anything regularly required.
If you do a web search under "reverse osmosis" you will find all the info you need about how they work.
Leroy Merlin do a few variants - see
http://www.leroymerlin.es/fp/12878754/equipo-de-osmosis-5-etapas-bbagua-eur-50 for an example. On continuous use it's recommended you change the three filters and post filter annually and the membrane every three years. The last cost for me for all the filters was €13.50 and the membrane was €30.15. All from Brico at the GA Centre
I originally had the separate tap but have since replaced that with the combined tap as shown as an additional item on the Leroy Merlin page.
As I posted before there is an initial outlay but after that the running costs are very low; and you never run out or have to carry loads of bottles.
Our water is direct fed and not off a tank so we don't have the issues that others refer to. Personally I wouldn't be without the filter.
Dave