This a pretty good site,lots of useful info:
http://www.ionowners.com/index.php
Like Swisstony has said,use circlip pliers,failing that then try 2 screwdrivers,one forcing the circlip out of the groove,the other pushing it upwards,do this at the same time if possible.Its not pretty and you may mishape your circlip doing this.
Or failing that bring it round to my house and i'll remove and re-fit your new one