It's still somewhat of a mystery. I'd tend to agree with the earlier gentleman
who called it a Bewick's Wren. They are fairly loud and would repeat the song
about that number of times. They don't normally like to go high in trees unless
they are well concealed, but your hawthorn and holly both are thick enough to
do that (since you didn't see the bird). And identifying a strange call as a
Bewick's Wren is a pretty good cover for my ignorance, since they do make a wide
variety of sounds, Make sure that you listen to several different tapes/CDs to
get an idea of the variety of their calls.