You can buy the tickets separately.
Such as
from Exeter St Davids to London Waterloo
or
London Terminals to Exeter St Davids.
Date:
28 March, 2010. From Exeter to London Waterloo, 5tickets, 8.6 pounds/ per ticket
7 April, 2010. From London Paddingtn to Exeter, 5 tickets, 7.39 pounds/ per ticket