Wolfgang Schwanke wrote:

The reason I'm asking is I'm having second thoughts now. It looks very
unspectacular, really just another platform in the main railway
station with LU logos. They could well have relocated the platform
used by Underground trains in all those years. Or is it still the
original place?

Yes and no. One face is still the original.

When the line was built there were connections with the national rail
network, for through running from the Great Western suburban stations. The
four platforms in the suburban station were allocated slightly differently.
In the 1960s the national rail connections were removed and the opportunity
was taken to alter the platform allocations, so that the Underground
platforms were entirely separate from the suburban platforms. This was
effected by moving the Underground (which used to use the middle two
platforms) across by one track, so that it used the two faces of the
northernmost island platform - the present arrangement. Cutting back the
suburban bays slightly also allowed a walkway from the end of the
northernmost mainline platform onto the suburban platforms to be created,
reducing movements on the footbridge.