Kitchen tap problem - query from a girlie!

[justwhackit]

Wonder if anyone can help a damsel in distress.
I have a tap as detailed in link below;

http://www.bristan.com/WebRoot/BristanDB/Shops/Bristan/Products/CS DRAWING-1028336-D2.PDF

Both taps now have to be turned on fully before any water emerges - hot more than cold - has been getting worse over last few months.

Am just about competent enough to dismantle them but not sure what I should be checking first. Got as far as looking at the two small circlips - one of which is kaput.
Can anyone direct me to which bits in the diagram I should be ordering, as circlips on their own don't seem to be an option.
Many thank yous

 

[JPC]

both hot and cold affected ?..poss debri in the tap...can you take the spout off and gently open hot or cold....see if anything shows...sometimes u can get crap in the pipework.