What is this pink flower

What is this pink flower

Observation - What is this pink flower. Description: This pretty pink flower is to be found along the edges of the Linford Brook at the moment. I'm sure it's something quite common, but I can't find the name.

This pretty pink flower is to be found along the edges of the Linford Brook at the moment. I'm sure it's something quite common, but I can't find the name.