Unidentified birdsong?

Dear all,

I would really love to identify the bird producing this amazingly varied song high up in a tree in our garden. I have a feeling this is a real amateur's question as I think it is probably quite a common bird, but I don't know what it is and couldn't see it well enough in the dark. 

Here is a video I took this evening  - apologies for the screeching blackbirds either side who decided to start up just as I began filming, but you should be able also to hear the bird in question a little further away in the tree. I was trying to look in binoculars at the same time, which explains the slightly wonky footage!

Many thanks in advance to anyone who can help shed some light...