Book two singles rather than a return.
Once you get past 7.30pm the price decreases substantially. The peak time on that route is 2.30 - 7.30 so you pay a higher price if you travel in that time. I normally caught the 7.30 train when returning from London, and the outbound 12.55 or similar when going from Broadmarsh. So I'd say look at the 7.30 train. But bare in mind everyone will be doing the same so turn up early and grab yourself a good seat before they all get taken. You quite often get people cramming in the corridors sitting on the floor because the train gets so busy.