Just been to Robert Dyas in Oxford. They have reduced the double life (ie 2 month ones) to half price in there triple packs to £7.49.
I originallly bought the Optima kettle which came with 6 of these filters - thay have lasted well over 18 months!
No more scale scum on your tea!!
Have checked - it is not this price online but know they are running this nationwide.