Alex wrote:

Does anyone know whether I can (legaly) travel through Richmond
without buying an extension, since my starting point & destination
are bot covered by my travelcard (and I don't pass any ticket gates,
just walk from one platform to another)?

You would pass a gateline, in the corridor connecting platform 1 (the
westbound SWT platform) to the rest of the station.

You can get across the bridge and down to platform 2 and then across to
the Silverlink and District platforms without passing through any gates.
--
Richard J.
(to e-mail me, swap uk and yon in address)