AFAIK nobody 'our end' knows how much of the T5 area will be in use at that point.
Estimates put it at an extra 30 mins walk each way (to the base of the tower) if you don't get delayed by security.
And that's parking in central area.
It will be interesting the first time that there are long queues at staff search and people start going home at 1430 without the afternoon watch in. When was the last time Heathrow closed?
