Seems slightly presumptuous to guess (as if I'd know); but it took me ages (and much "head-scratching") to realise that Ravens have an incredible vocal range, some of their calls perhaps matching your description.
Thanks for that everybody. Unfortunately not a Scops. The style of the call is very similar, the right sort of pitch and length of note and time between them, however the note is fuller. probably quite a lot louder, and from the clip that you directed me to, probablycarries further.
It has not called since my original post.
Thanks again anyway