Bird song identification

Good evening 

I live in Kent, and earlier this evening before dusk, I heard a bird making an unusual sound that I don’t think I’ve heard before. I didn’t manage to get a recording but it was making a sound like 

ka ka kakaka 

It just get repeating this sound. I have a bird song identifying app, but I wasn’t close enough for it to identify the sound, and I was wondering if anyone could help? Thanks