Help identify bird call

I had the (mis)fortune of being woken up at 6:00 this morning by a bird call, which I'm struggling to identify.

It made 3 to 5 evenly spaced (c. 0.5 seconds), staccato high pitch noises, waited around 3 to 5 seconds, then repeated. It was a harsh, non-variable tone. I actually thought it was an alarm to begin with, but there were slight changes to the tempo and a variable number of calls.

Would appreciate any help trying to ID.

I'm in London - sorry I didn't get a recording :(