In message .com,
David of Broadway writes

And why is this outage taking place in the middle of the week
(immediately following a holiday weekend, yet!) in the first place? At
first I thought it was necessary to allow the line to Terminal 4 to
reopen on 1 September. But according to the latest batch of posters,
the line to Terminal 4 isn't reopening until later in the month!

New timetable (including the return to T4) comes in on 17 September.

The 3 day closure is to allow all the new signalling to be installed and
tested as much was disconnected and removed when the loop was closed.
This also includes the extra stuff for the T5 extension.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)