Gary, I've looked up the train on the site but I'm being quoted about £18 each way from Victoria. This ain't right is it? What times are they classing as off peak? And I live in Zone 1 so I'm only selecting London as my 'from' station.
I just tried again and it came up with £10.85 open return.
It will be cheaper if you travel with 4 or if someone had a gold card you're with.
go to their website - click on 50% offer - then 'book your half price tricket' - choose saturdays date - after 10am - choose open return - unclick 1st class and it should give you decent prices.