It's still somewhat of a mystery, but I'd be inclined to agree with the earlier
gentleman who called it a Bewick's Wren. They're somewhat loud, and they
would repeat the call several times. They don't really like to get high in
trees unless they're well-concealed, but your hawthorn or holly are both
obviously
thick enough to hide them (since you couldn't see the bird). Identifying a
mystery call as a Bewick's Wren is also a fairly good refuge for my
ignorance, since they do make a surprising number of calls. Make sure that
you listen to several different tapes if you can, to get an idea of the
range of their calls.