Strange bird, never heard it before

I heard a new bird call when I was outside at dawn a few days ago. I found the bird sitting in an ash tree on the edge of farmland. It was about jackdaw sized and I think blackish, but the light was poor. In fact I thought it was a jackdaw (and not the one responsible for the strange call) until I was seeing it from a better angle and realised that it’s beak was all wrong for a jackdaw. It’s beak was much smaller than a corvid’s, quite small and neat compared to the bird’s size. It wasn’t hooked either. It’s neck was not slender like a pigeon, but more like a jackdaw. It was sitting with its back quite horizontal to the branch, rather than upright or angled, like most birds, which looked unusual to me. It was fidgeting from food to foot and stopped calling when I got close, but didn’t fly away. It’s call was either one or two notes like the “who” of an owl, but much more tuneful and rounded-sounding than an owl. Plus it’s face was not flat, but shaped like a blackbird. There were long gaps between each call. I was so puzzled that I forgot to get my phone out and record it, sorry. I’ve been back every morning for days, but  can’t  find it. The rspb bird identification tool hasn’t helped, despite messing about with options for ages. Any ideas?