Very interesting. Various species of fish could easily get into this pond when flooding occurs and flow from the Humber washes fish and/or eggs into the pond. If conditions allow, they then reproduce and colonize the pond. That's my theory.
We've seen many such ponds contain Bass and other various species of fish, especially when the pond is located next to, or connected to a major river, creek or tributary.
Question for you, how long has this pond been there?