it's a wren, jan.
I think Lou has summed this up very well Jan, I can't hear anything within the recording which would suggest an aberrant Greenish warbler, or which would exclude Wren.
I agree with you with regards to the volume, all of the singing birds I've heard in Poland were fairly loud. I'm surprised that you've said there were lots on Hel during the spring passage - my understanding was that there are less than 50 pairs in Poland, and away from the south east, it's a scarce migrant? It is quite likely that I'm out of date though...
Greenish Warbler observations are on a sharp rise in the last years - not sure if due to an actual influx or due to a big increase of observers, I think both factors may be at play. Attached is the result for the search on Greenish Warbler in last two months on ornitho.pl (I think you need an active account regularly adding data to replicate that). In general, there were dozens reported by the various birder groups during the "rarity week" on Hel and I have heard the much more typical song at several places.
Thanks for that Jan, very interesting, very scattered records! Has the breeding population radiated out from the east as well?
Typical Greenish Warbler song is not too difficult to separate from Wren song, but the species mimics Wren song quite often. Then it is so similar that it can be mistaken. I think all mimickers I have heard have switched between Wren song and normal song (but did not check that from notes). The habit of mimicking Wren is interesting and strange. Both species breed often in similar habitat, but I it hard to believe in any inter-specific territoriality, because they are otherwise very different birds.