Perhaps most spectacular is the small patch of Flowering Rush (Butomus umbellatus) at the eastern end by the water. It's not just that it has survived, but that it has flowered reliably every year. The photo is not taken at Branton, but is very similar to that location.
In the grassy margins of the path on the north side of the east lake was another alien, a native of South East England. It's well worth looking out for if you are going round the ponds because you are unlikely to find it anywhere else in North Northumberland. The plant is Grass Vetchling (Lathyrus nissolia). You can see from the leaf shape why it gets its common name.