2 years worth of water filter cartridges for under £20, seems really good to me - I recently paid £11 for just 3!
Works out £1.66 each, or just 83p a month.
If you don't own a jug pick one up from argos for £4.99 (includes a 1 month filter) sku: 842/4578.
£24.98 with the jug for 2 years+ of filtered water - Bargain.
* 2 years supply of Aqua Optima 60-day DOUBLE LIFE filters
* Unique 5-step fast-flow filter system fits all Aqua Optima jugs
* Removes chlorine, limescale & impurities from tap water
* Provides cleaner, clearer water and great tasting hot and cool drinks
* More economical than bottled water