A cunning plan Sadie, except there is no communal way to get through block 6 like that. Your lookout/driver would need to have rented an apartment there to do that.
BTW, in this theory of yours, what was the motive of your perps?
Jeez, block 6 not block 4 ! Both Misty and I got it wrong.
*&*%£
My excuse is a senior moment. What's yours misty ?

Now I thought that there was a way thru. Yonks ago, someone somewhere said so, but I dont actually know if that is true.
How many flats are there on each floor?
Guess they could have rented a flat, or even opened a flat illicitly, if they have access to keys, as they seem to have.
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Reason: Abduction of a little girl, in this case to order.
The same as the string of abductions since 1991 , roughly one every two years., but not to order generally previously.
I would like to point out that this is not my main theory. None of this has gone to SY, but it did seem quite feasible... and still is with stolen or borrowed keys and illicit entry to a flat in the building.