How could I express this in a formula- I wish to add up the number of
instances that stock doesn't exist and no sales have been made, hence
a possible reason for no sales.

e.g.

Z8 = Sales in London which are 0
N46 = Stock for the London depot which is 0
Z12 = Sales in Manchester which are 20
N50 = Stock for the Manchester depot which is 0
Z16 = Sales in Bristol which are 0
N54 = Stock for the Bristol depot which is 0

The answer to above should be 2

I have a range of 30 locations, divided into 5 regions, but they are
not in a contiguous range