how can i fix my tap, aston matthews zyam range?

firstly the tap started turning a bit and not turning the water on but then when you pushed it further round the water would come out. Then, last time i used it i turned it and it did the same thing but then no water came out and the top came off. Does anybody know how i can mend it
(the link to a pic of this tap is http://www.astonmatthews.co.uk/contents/taps_pics/zyam1.jpg )

Thank you very much