OK - so there are only two - but I think the other two had been shot down! These balloons coupled with the security at Canary Wharf who told us we were not allowed to take photos, reminded me of this verse from the Nena hit '99 red balloons":

99 red balloons
Floating in the summer sky
Panic bells, it's red alert
There's something here
From somewhere else
The war machine springs to life
Opens up one eager eye
Focusing it on the sky
Where 99 red balloons go by