What is living in my chimney?

Hello all, 

Hoping someone can help me to identify what is living in my chimney. Over the course of a week or two I have only heard it at night - ranging from early morning to the dead of night. It is not a constant noise, nor a frantic one as if something is trapped. It comes in short bursts of anything from a few seconds to a minute. This usually continues on and off for around 10 minutes or so and then settles down with no noise for several hours. 

The noise is a high-pitched squeaking - too loud for a mouse but also not enough to be a large bird. Whatever it is it sounds like there is more than one of them and almost like they are fighting. It also comes with 'clunking', like wings beating against the wall. I can't make sense of it - it can't be baby birds because it's December and the wrong time of year to nest. Bats should be well into hibernation by now. Mice/rats would be down at the bottom of the chimney, but this is definitely at the top. 

Having done some research the closest answer I can come to is a Chimney Swift (https://www.youtube.com/watch?v=M7b8PpiNhJg), but these don't seem to be too common in the UK? Especially not at this time of year when they migrate south. But it certainly fits the bill. 

I have a recording which might help, captured last night at 2.35am. 

Thanks in advance, from someone desperate for a good night's sleep!!

3554.Chimney recording 1.m4a