Apologies for the faint recording, I just pointed my mobile phone in the direction of the bird. We're used to the mimicry and variety of Marsh Warbler here in May/June but this species comes a close second when it does this Autumn subsong. All the sounds you hear (apart from the cow bells nearby) were being produced by this individual, including the eerie owl-like gargle.
I made other recordings in a ten minute spell but cars or aircraft passing spoiled the result. Among the species I heard it mimicking were Black Woodpecker, Nuthatch, Chiffchaff and Blue Tit.
I only ever hear it doing this in September, what do you think it is?