SWT advance fares available for the trains from London Waterloo.
Waterloo to Exeter Central (since it stops there) for a whole whopping £13.
Return tickets on this route not available yet but I'll be dropping by to see parents near Crewkerne on the way back.
[EDIT] Return tickets available. £11 advance for Crewkerne to Clapham Junction. No advance fares for Exeter to Crewkerne so it's a choice of a £12.90 walk up fare or a 60km ride in the morning. In for a penny...