Newly installed kitchen tap has lost pressure but was fine before that for about 2 months after instalation. They are installed with pvc pipe not copper although there is a connection off the main(3/4") feeding into the house which is copper. ( has a special copper to pvc Tee installed to connect the two)

Have you removed and cleaned out the areator? That's the first suspect. Should simply unscrew. Watch carefully when you take it apart. I've done that and then not be able to remember how it goes back together.

Check this out: How to Unclog a Faucet Aerator For Dummies - YouTube (http://www.youtube.com/watch?v=e_DaEA5UtqY&feature=fvwrel)

No, I am not suggesting you are a dummy. That's just the name of the video.