Unidentified bird call

For several nights in succession we've heard this bird.  It sits in a tree in a small wood by our house, and it makes a repetitive squawk over and over again, every four seconds or so.  It is just one note, no variation.  I can stand beneath the tree (but it's too dark to see it) and it carries on regardless, nothing seems to make it fly off.  I have heard this for at least two hours, between 10 p.m. and midnight.

I have no idea what this is.  The birds we have around here are magpies, pigeons, greater spotted woodpeckers, crows, jays, the occasional sparrowhawk, tawny owls.  Plus the smaller garden birds.

Any ideas as to what this might be? 

Many thanks... I look forward to hearing!

Annie B.