No final date had yet been set, although August/September/October is likely.
With the adjustments to stabling arrangements from the new timetable on 15 May it could be feasible to operate the whole of the District/Circle/H&C with the S7s already in London, although no spares (except D Stock) or trains for routine maintenance would be provided.
All S Stock (S7 & S8) are required to workback to Derby starting in July/August, with 3-4 away at any one time, so cover must be kept for these until more trains are delivered, currently 23 trains outstanding.