mellors, this BBC site has some info on what to do, but it is mainly aimed at ppl being stalked on foot rather than by phone. There's also a BBC radio programme on it here. There is more info here too. This site talks about the 1997 Protection from Harrassment Act and the Yahoo directory lists various websites related to stalking.

Hope that's not too much info

Basically though what they say is that you should report her to the police and not encourage her in any way, and that you should not even talk to her.