From Wiki:

The Amethyst Incident, also known as the Yangtze Incident, in 1949 involved the British Royal Navy ship HMS Amethyst being trapped on the Yangtze River for three months, during the Chinese Civil War.

About the Memorial:

The circular planting of 46 Chinese euonymus plants commemorates each life lost during the Yangtze incident in China in 1949.

A plaque alongside the memorial tells the full story of the Yangtze Incident. Inside the circle are a black polished granite monument and trees planted as a tribute to HMS Amethyst, HMS Black Swan, HMS Consort and HMS London which were the four ships involved in the incident.

The human cost in the four ships was 46 dead and 68 injured. The tree planted just outside the circle remembers the RAF Sunderland flying boat named D-DOG ML772 from 88 Squadron that, on the fourth attempt, landed successfully on the Yangtze River alongside HMS Amethyst to deliver medical supplies and a replacement doctor.*

*From the National Memorial Arboretum Guidebook Edition 4