Barry Salter wrote:

The best bet for lengthening at South Tottenham would be at the East
end, but even then you haven't got much room to play with before the
junction.

Wouldn't staggered platforms work there - extend the eastbound platform at
the east end and the westbound at the west end? It wouldn't matter if they
extended beyond the junctions as the only trains stopping at South Tottenham
are those on the T&H line (in fact services from Copper Mill heading to or
from Seven Sisters can't stop as the up/down crossover is midway along the
platforms).