Mortimer wrote:

There will be a major blockade for three/four weeks at London
Blackfriars (mainline) within the next few years (in relation to bridge
works I think). Other than that, the next few years will involve the
closure of the core Thameslink route during weekends and, from December,
weeknights as well. It is possible that some of these closures will
require services to/from London Blackfriars to be bustituted.

_Thameslink 2000 environmental statement : main report – inner area
(June 2004)_

2.7.6 On the north bank, the existing separate Network Rail and LUL
ticket areas will be combined into a single street level shared ticket
hall, accessed from a new station entrance off Queen Victoria Street.
The existing subway entrance to the station will be closed. A new space
accommodating the entrance, ticket hall and concourse will be created
behind a façade rising to the full height of the existing building at
167-179 Queen Victoria Street, which will be demolished. This new space
will also contain an LUL ventilation shaft and a mezzanine deck but will
otherwise be open to its roof, creating a ‘cathedral entrance’ to the
station. Access down to the Underground and up to the mainline station
will be beyond a shared ticket line via escalators and lifts. The
‘cathedral entrance’ concept supersedes the design described in the 1999
ES, which envisaged a single storey replacement to 167-179 Queen
Victoria Street (albeit one making provision for subsequent development
above it) – the so-called 'missing tooth'. Retail units will be provided
at street level and elsewhere within the station.

_Thameslink 2000 Statement of Case (March 2005)_

4.26 The design of the remodelled Underground Station now involves the
complete demolition of the existing station, apart from the track and
platform edge. It involves the provision within the new station of new
escalators and lifts for those whose mobility is impaired, in addition
to replacement of the existing station facilities and construction of a
combined Underground/National Railway Network.

4.27 The closure of the Underground station during the period that these
works are carried out is necessary because the safety shield required to
separate the works from the running lines will reduce the available
platform area to such an extent that the station cannot be operated
safely for the majority of the construction period. Alternatives (such
as single platform operation and weekend only closures) have been
examined but all scenarios involve the reduction of the platform
available to below an acceptable standard of safety. The construction
strategy will be to close the station; get the work done in the shortest
possible time in the safest possible environment; and re-open to a
completed new station.

4.28 The closure would be for two years. A shield would be constructed
around the track and train envelope along the entire length of the
platforms. This would permit the works to construct the new station to
be carried out at the same time as the through running of District and
Circle Line trains.