Looking at the layout sketch in #4, it appears three of the four main lines will have to be re-aligned to make room for the platforms. This would most likely require a closure of the Slows for several days to slew them to their new alignment, followed some time later by a closure of the Up Fast (and probably the Down Fast too) to move that. Although some work can be done before and after these closures, but the lines have to remain useable up to the start of the closure and be useable again afterwards, so the closure must be long enough to achieve this. Although I believe the MML still runs as a two-track railway on Sundays, I doubt that would be long enough so these would be disruptive possessions with timetable changes, unless perhaps they could be done over a Christmas period or when the is some other work going on that would reduce the service. OLE re-tensioning is a collection of fairly short tasks, each repeated many times, so probably doesn't need such a long closure.