I had a look through the NR Conditions of Carriage, but couldn't find
a definitive answer to this one, so I thought I'd ask you
knowledgeable people :-)

Occasionally, I have cause to travel from Brentford to Watford
Junction (changing at Clapham Junction), with a pre-bought ticket. If,
for some reason, it happens to be more convenient for me to start my
journey from West Brompton or Olympia on a given journey, is my
original ticket still valid?