Hi Nick,
We deliberately took a sample that would be representative - so we split our customers into different groups by balances (as you rightly said - groups with > £1,000, groups with less) and then took the same % as a random sample from each group so that it best represented our customer base.
This was to give us a good idea of how different people would respond to being able to earn interest with us and get a feel for Monzo as a whole, whilst still being fair and picking the same % from each group 
Hope that helps!
Jack