An answer after travelling towards Hither Green (well actually to Waterloo and Central London) on Wednesday 8th August 2012.
In brief: IT IS ACTUALLY CHEAPER TO BUY RAIL TICKETS TO OBSCURE STATIONS IN LONDON SUBURBS ON THE OPPOSITE SIDE OF LONDON FROM WHENCE YOU COME, THAN TO BUY TICKETS TO 'YOUR' LONDON TERMINAL.
So a return ticket to Hither Green at the height of the Olympics, admittedly via the slow Salisbury route cost a Senior rail card holder £20.40; a similar return to Waterloo was about £25 and to Paddington about £30 - all advance tickets available two or three days before travel and arriving in London after 11.00 and getting back to Bristol after 22.00.
Experiment on the First Great Western website.
