I just chatted with Bing again and found something.

Mandarin ducks were introduced to the UK from China and other parts of Asia, as their name suggests¹². The first recorded introduction was in the mid-18th century by Alfred Ezra at Foxwarren Park in Surrey³. However, it was not until the 1930s that escapees from captivity started breeding in the wild⁴⁵. They are one of the few introduced species not to cause any ecological problems, as their favoured habitats are not used by the UK's native wildfowl⁴..

Source: Conversation with Bing, 16/05/2023
(1) Mandarin Duck Facts | Aix Galericulata - The RSPB. https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/mandarin/.
(2) Mandarin duck | The Wildlife Trusts. https://www.wildlifetrusts.org/wildlife-explorer/birds/waterfowl/mandarin-duck.
(3) Wildlife - Mandarin ducks | Great British Life. https://www.greatbritishlife.co.uk/things-to-do/wildlife/22631444.wildlife---mandarin-ducks/.
(4) Mandarin Duck | Bird Identification Guide | Bird Spot. https://www.birdspot.co.uk/bird-identification/mandarin-duck.
(5) Mandarin Duck Bird Facts (Aix galericulata) | Birdfact. https://birdfact.com/birds/mandarin-duck.