Welcome to BF.
I see a bird flying fairly directly and disappearing behind the trees: too tiny/distant/brief to do anything with. Don't hear anything raptorish - and separating harriers on voice would be a new one on me - in fact, hearing a wandering/migrating harrier call at all would be a new one on me as well. I could well imagine that individual variation in harrier calls would swamp any diagnosable interspecific differences. I don't know what the foreground call is (sounds a bit like a distant macaw!), but I doubt it's the flying bird as it sounds too close and the volume is constant and not decreasing with distance. (Actually, a macaw would likely fit your description better than a harrier, especially the tail - and what's visible of the bird's flight in the vid does look more macaw than harrier - direct and fl
Sounds good for a macaw and bad for a harrier. A harrier that's in direct flight over country that's not suitable for breeding is not going to be calling or displaying. The call you've linked to doesn't (to me) sound anything like the call on your video.
I've done a bit more searching on the web and think you're right. There's a place called Larmer Tree Gardens not far from where we were and they apparently have free-ranging macaws so I think it must have been that. Would never have thought of it. The flight profile did look much more like a harrier than a macaw but the call, location and probabilities outweigh that. Thanks again for your help - much appreciated.