"Richard J." wrote
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.

That's not quite right. The Underground used the *outer* two platforms (13
& 16) but now use the northernmost island (15 & 16).