Thanks. Is it possible to excess the outbound portion only of a SSR to a SVR? I recall that it can be done on the return leg.
When you excess a return ticket to travel one leg by a more expensive route, you only have to pay half the difference and you can excess either the outward or the return leg.
But when you excess a return ticket to travel one leg at a more expensive time, you have to pay the full difference in price, eg. the SSR becomes an SVR ticket.
For Cardiff - London Paddington, the 2010 prices appear to be:
SSS £35.00
SSR £60.70
SVS £42.50
SVR £74.00
Note that in 2010 this SVR is less expensive than SSS + SVS. This was not the case in 2009.
You say that this is a day trip, so I should also mention that there are relevant Travelcards:
Cardiff - London Zones 1-6
Super Off-Peak Day Travelcard £66.00
Off-Peak Day Travelcard £79.00