Peter Smyth wrote

Conditions of Carriage 43 says

43. Help from Train Companies if you are stranded
If disruption caused by circumstances within the control of a Train
Company or
a Rail Service Company leaves you stranded before you have reached
your
destination and the Train Company whose trains you are entitled to
use
is unable
to get you to that destination by other means, any Train Company
which
is in a
position to help will, if it reasonably can, either arrange to get
you
to that
destination, or provide overnight accommodation for you.

So in this situation Southern already has a duty to take the
passengers
and then sort it out with FCC later.

I am not sure that CoC 43 means what you think.

"stranded before you have reached your destination" need not include
"unable to start your journey", hence the reference to overnight
accommodation, nor is it really clear that it includes inability to
start a return jouney with a return ticket. Also the drafter of CoC 43
probably didn't envisage the case, now common, of buying two singles
for a return journey.

--
Mike D