Andy wrote:
I understood it was because the GPS system, which controls the
selective door opening, can't see any of its satellites when it is
under the building built on top of the platforms. The conductor has to
wait for the system to time out allowing it to be overriden. It only
really seems to affect the Southern platforms, the South-Eastern
platforms (1-7) generally seem to have normal opening times.

However Southeastern's 375s have similar problems at Cannon Street,
although they've been better of late, perhaps because the office
buildings above the station are being redeveloped so maybe the GPS
signals are getting through now?

--
Roy