"John Salmon" wrote

So what route (i.e. which right-hand crossovers) did fast services (in
both
directions) take between the Quarry lines and the former "through"
platforms
(4 & 5?) at East Croydon? I had assumed the move was made at Stoats Nest,

That is correct. When the layout was rationalised, one of the changes was
relaying the Stoats Nest crossovers. Previously, travelling south, they ran
from the old Through Lines (eastern pair) to the Quarry Lines. Afterwards
they ran from the new Fast Lines (western pair) to the old line through
Redhill.

Peter