Yep, had a problem too. After descaling to head, I struggled too, not helped by the fact that the whole thing exploded when I took it apart so I could sus out exactly where it all came from, and went.
The answer. .....Get all the parts of the shower head infront of you with the main part on the left and work down to the black rubber flanges on the right and finally the springs to the far right. Working from left to right, scoop the row of parts with the left hand until you have them all in your grasp. With the right hand, open the dust bin top open and then deftly chuck all the parts therein. Then, grab your car keys and go and buy another Mira shower head at £30 quid. I hope this is of some use to you. Nice to hear from a fellow share holder of Mira!!
Far Off Fawcett Majors (problemo), January 2010