Not even sure this is a bird but it comes from a large oak tree in rural lincolnshire, at night. Very regular and constant.

I think it's probably the begging call of a young Tawny Owl. Here's a recording of one for comparison purposes.

Ireneusz Oleksik, XC649893. Accessible at www.xeno-canto.org/649893

We've had quite a few queries about calls like this recently.