I'm in the same position as you. Haven't received anything since accepting my provisional offer.
I only put down Stratford for preferred location. But a friend of mine applied for a role internally, only put down stratford for location preference, got offered the job but in Croydon - so he turned it down. So they might not be able to cater to your preferred location even if you only put one choice down.
Apparently they have a bigger centre in Croydon ready for all these new recruits whereas they only have a very limited handful of vacancies in Stratford. Also, rumour has it that they are looking to have an office in Canary Wharf until the big move to the new Stratford offices in 2020.