It seems to be easier to buy them on the day. There's instructions on the ticket machines or they can be bought over the counter at the ticket office. If you buy at the machine, go to the 'destination finder', type in London, select the option for London Terminals, select 'SWT Only' and it will...
SWT have extended their £10 return to London at weekends from anywhere on their network until 16th March, presumably due to the weather being rubbish.
Good news.