Westport pier was located in Westport.
It was the pier from which Sarah Jane Smith's best friend Andrea Yates fell into the sea and drowned on 13 July 1964. Sarah Jane witnessed the event.
The Trickster used the event to create chaos, attempting to coax Andrea into switching places with Sarah Jane, and thereby creating an alternate timeline.
Just before she fell, Andrea's last words to Sarah Jane were: "Remember me". Sarah remembered Andrea as "the friend who helped [her] save the world". (TV: Whatever Happened to Sarah Jane?)