Hello again, Mart
thank you for your fast reaction!
The pictures show a Chiffchaff, please note:
- the right Phylloscopus like built of a small rounded warbler with a normal looking tail, bill and so on ... Nothing special with this.
- colouration consists of olive, dark-greenish and yellowish tones. To describe them as drab and dirty would surely come into my mind, but it would be unfair to this bird (I hope thats understandable)
- good head pattern with a broken eye-ring, whitish in the lower half
- an all blackish bill and dark legs (many Willow Warblers have just noticable stronger legs, enhanced by plae colouration. but far from all)
- a short pp
Conclusion? Your Chiffchaff is (nearly) perfect, so it has to be one (and a Willow Warbler can be excluded with confidence, I think).
And the song? Maybe you heard a different bird? I am looking forward to hear the recording, I hope you can get one! As said, there are mixerd singers, maybe it is indeed one of those. Examples are here:
www.xeno-canto.org
Yes, many mixed singers often look more like Willow Warbler, but as said not your bird, that looks like a Chiffchaff imo to an extent, that I am sure it is indeed one despite you described a strange song.